and vol. vii. p. 23.) My words are "most liquids

except." Those expressions, are, I believe, not generally deemed synonymous. My experiments were much more numerous on water than on other liquids, and what I had more particularly in view was to show that any slight modification of water by acids, salts, &c., such as might naturally occur, was not sensibly distinguishable from pure water in regard to absorption. On concentrated liquid acids and saline solutions I had not made many experiments. A strong solution of common salt I found to absorb only one third of the volume of any gas that water did, and this was the reason of my saying "most" instead of "all liquids except;" but guarded as the expression was, I am ready to allow that it was not sufficiently so. sure's copious experiments on other liquids than water far surpass mine in number and variety, and will be found a valuable acquisition, if the accuracy of the less absorbable gases be equal to that of the more absorbable ones.

Saus

If the influence of chemical affinity did not exist, the gases would be absorbed by all liquids in the same order, according to Saussure; and finding them not so, he concludes, that the absorptions are occasioned by affinities. A better distinction in my opinion would be, that if a volume of any gas or mixture of gases is absorbed by water in proportion to the pressure of the incumbent gas, and the same volume is capable of being expelled again unchanged by the usual means of boiling, the air-pump, or agitation with any other gas, then the absorption is mechanical; but if a change in the quantity or quality of the gases expelled be observed, it must be ascribed to affinity: thus when nitrous gas or sulphureted hydrogen are pressed into pure water freed from all air, we can rarely if ever recover the same quantity again, the respective gases being in a short time partially decomposed. If it be said that solutions of ammonia, muriatic acid, &c. in water, must upon these grounds be considered as mechanical combinations; I grant they are combinations of a mixed nature, partly mechanical and partly chemical. The immense condensation of volume of those gases by water cannot be accounted for on mechanical principles alone; the water must have an affinity for the bases of these gases, or for their caloric, or both, and besides the quantity is not as the pressure. But when no condensation of gas takes place, and the quantity is accurately as the pressure, to call this a case of affinity seems to me just as reasonable as to ascribe the air in a sandhill to the chemical affinity of sand for air, and to argue that that affinity varies according to the state of the barometer.

་

It may not be amiss to sum up these remarks under a few heads, exhibiting the leading principles of the theory of absorption which I adopt, in order that they may be more clearly understood. The gases are of course chiefly those of which water does not take more than its bulk.

1. The quantity of any pure gas which water absorbs is in proportion to the pressure or density of the gas.

This was Dr. Henry's discovery; but I adopt it as an essential principle of the theory. Saussure also confirms it.

2. The quantities of any mixture of gases which water absorbs are also in proportion to the pressure or densities of the several incumbent gases after the absorption has ceased, (but not in proportion to their pressures before the absorption, unless these twa ratios happen to be the same); and are the same as if the gases were alone, allowing for the diminished density.-Thus, water charged with atmospheric air of unlimited volume contains of a full charge of oxygen, and 72 of a full charge of azote; but if water be charged with a limited volume of air, as, then it will contain less oxygen and more azote than specified above.

00

This was a discovery of mine; it is confirmed by the experiments of Henry, and by the four experiments of Saussure above explained, the only ones of his that apply to it.

3. Heat and cold, or change of temperature, has no influence on the quantities of gas absorbed by water.

This was an observation of mine; the idea was at first suggested by a consideration of other facts, and afterwards confirmed by experiment. As heat increases the force of the incumbent air in proportion as it increases that of the air in the water, the equilibrium is not disturbed The reason why heat seems to expel air from liquids is that it generates steam, which removes the atmospheric air from the surface. The air-pump, or hydrogen gas, will remove the pressure of the azote and oxygen of the atmosphere, and are equally efficacious in expelling the air from water without heat. This feature of the theory, as has been observed, has not been noticed by Saussure.

4. The quantities of the several gases absorbed by water are as 1, , &c.; the volume of water being unity.

This observation occurred to me during the investigation, and I attempted to show that these proportions necessarsly resulted from the preceding phenomena. Though many of Saussure's results differ widely from the above proportions, in consequence of their being erroneous as shewn above, yet it must be allowed that some exceptions occur in regard to this law, particularly in the class which should be absorbable: whether the approximations are accidental, or whether they are founded on the principle of equilibrium I have suggested, may be a fair subject for future discussion when the facts are ascertained beyond doubt.

The assertion that I made, that "most liquids freed from viscidity, such as acids, alcohol, liquid sulphurets, and saline solutions in water, absorb the same quantity of gases as pure water, except they have an affinity for the gas, such as the sulphurets for oxygen, &c." appears to me to be too general and comprehensive. Saussure has clearly shown that oils, acids, alcohol, and saline solutions differ very materially from waters in the quantity of gases absorbed ; but for any thing that appears these liquids all agree with water in the other three primary laws.

I must now leave it to be determined whether "it would appear from these experiments of De Saussure, that Mr. Dalton's theory [of the absorption of gases by liquids] is erroneous in every particular." I remain respectfully yours,

JOHN DALTON.

ARTICLE VI. ·

Defence of the Objections to Prevost's Theory of Radiant Heat. By John Murray, M. D. F.R.S. E., Lecturer on Chemistry in Edinburgh.

SIR,

(To Dr. Thomson.)

Edinburgh, Jan. 20, 1816.

In the general view you have given in your last Number of the late improvements in science, you mention that Mr. Davenport had written a very complete refutation of some objections that had been started against Mr. Prevost's theory of Radiant Heat. This refers, I believe, to the answer given by that gentleman to some objections to the application of that theory to radiant cold. One of these objections I had advanced, and I now take the liberty of making a few observations on Mr. Davenport's reply to it, which I was prevented by circumstances from doing at the time it was published.

When a tin cannister containing a freezing mixture is placed opposite to a reflecting metallic mirror, in the focus of which a thermometer is placed, if one of the surfaces of the cannister be covered with a coating of lamp-black, it is known that the depression of temperature which is indicated by the thermometer, is much greater than when the clear metallic surface is opposed. This appears to me inconsistent with Prevost's explanation of radiant cold. That explanation assumes that the effect depends merely on the interchange of rays of heat between the thermometer and the cold surface, regulated by the mirror, the thermometer being at a higher temperature, and therefore giving off more radiant heat than it receives in return from the cold body; so that its temperature falls. Now it seems obvious that of different surfaces giving off different portions of calorie by radiation at the same temperature, the one which gives heat will allow of the greatest depression of temperature in the thermometer, for it is the one which will make the least return. A metallic surface is that which radiates least, it therefore should cause the greatest degree of cold when opposed to the thermometer; but it causes the least, and the blackened surface which discharges the largest quantity of calorie by radiation is the one which, in this experiment, causes the greatest depression of temperature in the ther

mometer,

Mr. Davenport's reply (which has been considered as satisfactory

by Mr. Prevost) is that the effect depends on the power of the different surfaces in reflecting radiant heat. Of the heat radiated from the thermometer a portion is always returned by reflection. A blackened surface, it is remarked, radiates much, but it reflects little; while it intercepts radiation or reflection from behind. A metallic surface radiates less, but it reflects as much as it fails to radiate; hence it is inferred, that by reflecting so much heat, though it radiates so little, it is powerful in counteracting the fall of the thermometer.

In this reasoning it appears to me that too little effect is ascribed to radiation, and too much to reflection. The comparative powers of different surfaces in producing the phenomenon of radiant heat show how much more influence is due to the radiating than to the reflecting power; the blackened surface which reflects scarcely any producing, when opposed to the thermometer, the greatest heating effect, because it radiates most; while the metallic surface, which returns the largest quantity by reflection to the thermometer, still produces the least heating effect, because it is inferior in radiating power. In the experiment with radiant cold, the same difference of effect ought to take place; the blackened surface, though reflecting little, still by its superior radiating power ought to produce the greatest heating effect, so as to counteract the fall of temperature in the thermometer; and the metallic surface, though it reflects best, yet emitting so little by radiation, ought to produce the least heating effect, and therefore admit of the greatest depression of temperature in the thermometer, all of which is the reverse of the fact. It seems to me, therefore, that the original argument is still just, and that, according to Prevost's hypothesis, the blackened surface ought to be least powerful in producing radiant cold. Or if even the circumstance of its inferior reflecting power should so far counterbalance its superior radiating power as to render it equal to the other, still no cause can be assigned for its cooling agency being so greatly superior. "A blackened surface," says Mr. Davenport, "radiates much it is true, but it intercepts an equal volume of radiation or reflection from behind; a polished surface radiates less, but it reflects as much as it fails to radiate." Its power, therefore, ought to be the same as that of a surface reflecting little; but which radiates as much as it fails to reflect; that is, the power of the two surfaces ought to be the same, and there is no cause why the blackened surface should be so far superior to the other in producing cold.

I am Sir, yours respectfully,

J. MURRAY.

P. S. I take the liberty of pointing out a slight oversight in the statement in your last number (pages 43 and 44) with regard to my paper on Mineral Waters. It is mentioned that, in the opinion I had advanced of muriate of lime and sulphate of soda being present together in a mineral water, I had been anticipated by Pfaff, who had stated muriate of lime and sulphate of magnesia as

ingredients of sea-water; and as his dissertation was published in Schweigger's Journal, September, 1814, the anticipation, it is remarked, is at least that of a year, Though the volume of the Transactions of the Royal Society of Edinburgh, in which my paper appeared, was published in June, 1815, yet the paper was read on the 20th of November, 1814; and the analysis itself was executed in August and September, so that, strictly speaking, there cannot be said to have been any anticipation. Besides, there is no novelty in the mere observation that muriate of lime, and sulphate of magnesia, or sulphate of soda, may exist together in a mineral water, for this has often been advanced, and has always been ascribed to the circumstance to which it is referred by Pfaff; the state of great dilution. The novelty of opinion consists in the inferring that these salts are the ingredients of a mineral water, from the obtaining by its analysis muriate of soda, or of magnesia, and sulphate of lime, and of course, regarding these latter not as original ingredients, but as products of the operation. Of this Pfaff seems to have had no idea, and could not indeed have had, as the account which he gives of the composition of sea-water is incompatible with it. He states sulphate of lime as an ingredient, as well as muriate of lime, which he would not have done, had he had any conception of the above opinion. His statement of the composition of a mineral water, which you give in the same page, is equally incompatible with it. The ingredients of that water, according to the view I have given, are carbonate of soda and muriate of lime, and not, as he states them, carbonate of soda, muriate of soda, and carbonate of lime.

ARTICLE VII.

Proceedings of Philosophical Societies.

ROYAL SOCIETY.

ON Thursday the 25th of January a paper by Sir Humphry Davy was read, containing further experiments on the effect of wire sieves to prevent the combustion of gases from passing through them. A sieve formed of wire th of an inch in diameter, and containing 10 wires in the inch, prevented the combustion from penetrating; but when agitated in an exploding mixture explosion took place. The explosion likewise took place when the wire became red-hot. When there were 14 wires in the inch agitation did not occasion an explosion. With 24 wires to the inch, the mixture did not explode even when the wire became red-hot. The author accounts for these singular phenomena in this manner. A red-hot wire of a considerable size is required to produce an ex