ON ARTIFICIAL REFRIGERATION. 919 



are only distant stages of the same condition of matter and are capable 

 of passing into one another by a process of continuous change. Con- 

 fusion lias arisen in the use of the almost interchangeable words gas and 

 vapor. Ether in the state of gas is called a vapor, whereas ammonia and 

 sulphurous oxide in the same state are called gases; yet they are all 

 vapors — the ether from a liquid boiling at 35°, the sulphurous oxide 

 from a liquid boiling at —10°, and the ammonia from one boihng at —37°. 

 The distinction, says Dr. Andrews, is thus determined by the trivial 

 condition of the boiling-point of the liquid, under the ordinary pressure of 

 the atmosphere, being higher or lower than the ordinary temperature of 

 the atmosphere. Such a distinction may have some advantages for 

 practical reference, but it has no scientific value. The critical point 

 of temperature aflbrds a criterion for distinguishing a vapor from a gas, 

 if it be considered important to maintain the distinction at all. Many 

 of the properties of vapors depend on the gas and liquid being present 

 in contact with one another; and this we have seen can only occur at 

 temperatures below the critical point. We may accordingly define a 

 vaiwr to be a gas at any temperature under its critical point. Accord- 

 ing to this definition a vapor may, by pressure alone, be changed into a 

 liquid, and may, therefore, exist in presence of its own liquid; while a 

 gas cannot be liquefied by pressure — that is, so changed by pressure as 

 to become a visible liquid distinguished by a surface of demarcation 

 from the gas. If the distinction be accepted, continues Dr. Andrews, 

 carbonic acid will be a vapor below 31° C, a gas above that tempera- 

 tiu'e ; ether a vapor below 200°, a gas above that temperature. 



The fact that Jacob Perkins, who had designed the first practical freez- 

 ing-machine, had liquefied gases and probably atmospheric air has not 

 met with the attention it deserves, and believing, as I do, in the true 

 genius of this admirable observer, I perhaps attribute more importance 

 than others might to the following note from Faraday's Experimental Ee- 

 searches in Chemistry and Physics. He says: "As my object is to draw 

 attention to tlie results obtained in the liquefaction of gases before the 

 date of those described in the Philosophical Transactions for 1823, 1 need 

 not, perhaps, refer to the notice given in the Annals of Philosophy, jS". 

 S., VI, 00, of the supposed liquefaction of atmospheric air by Mr. Per- 

 kins, under a pressure of about 1,100 atmospheres ; but as such a result 

 would be highly interesting, and is the only additional one on the sub- 

 ject I am acquainted with, I am desirous of doing so, as well also as to 

 point out the remarkable difference between that result and those which 

 are the subject of this and the other papers referred to. Mr. Perkuis 

 informed me that the air upon compression disappeared, and in its place 

 was a small quantity of a fluid, which remained so when the pressure 

 was removed, had little or no taste, and did not act on the skin. As far 

 as I could, by iuquiry, make out its nature, it resembled water; but 

 if upon repetition it be found really to be the product of compressed 

 common air, then its fixed nature shows it to be a result of a verj- diifer- 



