918 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 



The energy of the rectilinear movements, that is to say, the mass of tho 

 gaseous molecules multiplied by the square of the velocity, gives the 

 measure of the temperature." The law of Boyle (1CG2) is as follows : The 

 volume of a gas varies inversely as the pressure ; or, in other words, the 

 pressure of a gas is proportional to its density. Cohesion, or the 

 tendency to molecular aggregation, which is so strong in the more hque- 

 fiable gases, causes deviations from the law of Boyle, especially near the 

 liquefying point. This cohesion is interfered with materially by the ad- 

 mixture with a condensable gas of one of the, hitherto called, i)ermanent 

 gases. For instance, if air becomes mixed with ammonia or even with 

 ether vapor, the pressure at which the gas liquefies is greatly increased. 

 The presence of air, or of some one of the more incondensable gases in a 

 freezing-machine, interferes materially with the changes in physical state, 

 so essential to the operation of the machine. 



Heat absorbed also most naturally affects cohe-sionj as the temperature 

 of a gas rises beyond a certain well-defined limit for each gas, the mole- 

 cular movements trimniih over cohesion and liquefaction is rendered 

 impossible 5 as Dr. Andrews states, the critical point is attained. This 

 point has been called by INIedeleyeff the absolute hoiUng-point. Just as 

 the addition of heat activates the motion to such an extent that the mole- 

 cules cannot be brought to rest by any amount of superincumbent press- 

 lu-e that we can apply, so the opposite condition may be imagined when 

 all heat has been ab|?tracted. When heat motion ceases, this is the 

 absolute zero, — 4G0O.CG Fahr., or — 2730.7C. 



Dalton extended Boyle's law, and declared that if different gases, 

 which do not act chemically on each other, are mixed together, the i^ressure 

 exerted is likewise the sum of the separate pressures of the diferent gases, 

 but Dr. Andrews has shoAvu* that by mixing nitrogen with carbonic acid 

 the critical temperatiu-e is lowered, and that Dalton's law of density of 

 mixed vapors only holds at low pressures and at temperatures greatly 

 above their critical points. 



It is fifty-five years since Faraday (1823) gave precision to our know- 

 ledge concerning the effects of pressure and cold on bodies usually gase- 

 ous at ordinary temperatures and atmospheric i)ressure. ISTorthmore had 

 compressed chlorine into a liquid t in 1805-1806. Faraday condensed 

 chlorine into a liquid in 1823, and afterwards succeeded in liquefying 

 hydrochloric acid, ammonia , and other gases. He afterwards learned that 

 Monge and Clouet had liquefied sulphurous acid gas in 1800, and in 

 1821 Bussy accomphshed this at ordinary atmospheric pressure at 12° 

 to 150 below Cent. 



l!^atterer, of Vienna, compressed oxygen, hydrogen, and nitrogen to 

 3,000 atmospheres without effecting liquefaction. 



Dr. Andrews I has demonstrated that the gaseous and liquid states 



* Proceediugs of the Royal Society, 1875. 

 tNortlimoro, Nicholson's Journal, XII '^GS; XIII, 232. 

 t The Bakeriau Lecture, Phil. Transactions, 18S9. 



