306 Mr. Coleman and Prof. McKendrick [May 29, 



William Thomson's researches, and involved upwards of 1000 obser- 

 vations collected in 50 tables, demonstrated among other things 

 that air having a pressure of 100 lb. per square inch, when passed 

 through a porous plug so as to reduce its pressure to that of the 

 atmosphere, only becomes cooled to the extent of 1 • 6^ C, hydrogen 

 gas similarly expanded being heated 0* 116° C. 



Free expansion of atmospheric air under pressure through a 

 small orifice of any kind was similar in its results, which Mr. 

 Coleman demonstrated by expanding a cubic foot of air of several 

 atmospheres pressure into a glass reservoir or chamber containing 

 a delicate air thermometer, which was not in the least affected, 

 although the result was thrown upon the screen magnified and 

 illumined by a beam of electric light. 



It was otherwise, however, when the experiment was made as 

 originally conducted by Gay Lussac and Dr. Joule, in which case 

 the vessel containing the air being expanded became cold, and the 

 vessel into which the air flowed became hot, the one phenomenon 

 neutralising the other. 



Mr. Coleman showed by an arrangement of his air thermometer, 

 the index of which was projected upon the screen, that the vapour 

 which exists in an ordinary bottle containing a little liquid ether has 

 sufficient tension to cause friction and production of heat in escaping 

 through a narrow orifice. 



In this case the heat abstracted from the evaporating liquid was 

 compensated by the latent heat absorbed in conversion of the ether 

 liquid into ether gas + the heat caused by the vis-viva of the escaping 

 molecules coming into contact with the walls of the orifice and 

 afterwards with the external atmosphere. 



The apparatus used was called a " Tripatmoscope " and was con- 

 structed as follows: — 



a is a bottle of say i of a cubic foot capacity containing a small 

 quantity of liquid ether, and into the neck of which was fitted by an 

 indiarubber cork a glass funnel, c is a plug of boxwood cemented 

 into the funnel bowl, and having an annular aperture barely sufficient 

 to admit the thin copper cylindrical bulb d of an air thermometer say 

 J-inch diameter and 6 inches long and packed tight into the boxwood 

 by making the bulb a little sticky with mastic varnish and winding 

 round it fine cotton thread. On opening the cock x at intervals of a 

 few minutes, a marked fall of the water index lo occurred which was 

 magnified and illumined by a beam of electric light — indicating nearly 

 1° F. produced by friction of escaping vapour.* 



The statement that is met with in some text-books, that no cold 

 can be produced by exjianding air without mechanical work being 

 performed, is not strictly correct, as one fraction of a given volume of 

 air can be cooled by blowing the other fraction into the atmosphere ; 

 but all such methods of producing cold are, however, from an engi- 



* This instrument works best in an atmosphere of about 70° F., but with a 

 slight occasional agitation of the liquid ether gives good results at 60°. 



