TRANSACTIONS OP THE SECTIONS. 83 



combustion the water of the chloride-of-calciima tube was found to be quite free 

 from sulphurous acid. Although these analyses, in the absence of a vapour-density 

 or atomic weight determination, which the small quantity of the substance did not 

 permit, yield no definite formida, yet they and the previous experiments show 

 clearly that the body is not thioformic acid. It crystallizes readily m white shining 

 needles from hot alcohol, ether, acetic or formic acids, the alcoholic solution being 

 neutral to test-papers. The cr\'stals melt at about 120° C, and sublime unchanged 

 at higher temperatures, depositing in long silky needles, — are unacted on by hot 

 or cold hydrochloric acid, solutions of carbonate or hydrate of potassium and 

 .sulphide of ammoniiun, — are decomposed by nitric and sulphuric acids, yielding 

 a heavy white precipitate with nitrate of silver, but none with chloride of barium, 

 wiien diy have little odour, but in solution in formic acid a strong peneti-ating 

 sulphur smell. 



On the Therymd Effects of Elastic Fluids. 

 By Dr. Joule, F.R.S., and Professor W. Thomson, F.B.S. 



In the year 1844, Mr. Joule showed that the thenual effects of compressing au 

 elastic fluid and of allowing a compressed elastic fluid to expand, were to be explamed 

 on mechanical piinciples. He demonstrated by experiment that the heat evolved 

 by the compression of an elastic fluid was proportional and equivalent to the force 

 employed ; and 2nd, that the cold occasioned by the dilatation of a gas was in con- 

 sequence of heat turned into work. He also snowed that if the dilatation of a gas is 

 managed so as to give out no external work, no sensible thermal effect is produced; 

 Professor Thomson showed that these residts were probably only approximate to 

 the truth, and would differ from it in proportion as the gas did not observe the so- 

 called gaseous laws, and he devised the plan of experimenting, which the authors 

 have since carried on in concert, in order to show the small but certain thermal effect 

 of expanding elastic fluids without giving oiit work. The method the authors 

 employed is to allow an elastic fluid confined at high pressure to escape through a 

 poroiis phig. It is obvious that if the gas obeyed the gaseous laws accurately, no 

 change of temperatmre would be occasioned by this process, for the cold of dilatation 

 would be exactly balanced by the heat arising from the friction of the air in the 

 plug. This is evident from the circumstance that the product of the pressure 

 through the space would be the same on bofh sides of the plug. Their first expe^ 

 riments, on a very small scale and with a veiy imperfect apparatus, decisively ex- 

 liibited a lowering of temperature of air on passing through the plug, thus showing 

 a non-observance of exact gaseous law, which was with difficulty detected by Reg- 

 nault by the use of a very elaborate and costly method, only applicable to certain 

 gases imder peculiar conditions. 



The method they employed, though so extremely simple, required several pre- 

 cautions. In particular it was requisite to employ a porous plug of considerable 

 thiclmess; for if a thin one was employed there was a rapid conduction of heat from 

 the high- to the low-pressure side, and also an irregular effect arising from the 

 action of numerous jets of air instead of a tranquil flow on the low-pressure side. 

 Hence they fomid a too large cooling effect when a diaphragm of leather was used, 

 in which case even hydi'ogen showed a slight cooling effect. 



The phenomena of a jet of air are highly interesting. Issuing at a high velocity 

 from a vessel in which it is confined at high pressure, its actual temperature may 

 readily be made 200° below the zero of Fahr. But this very low temperature 

 cannot be easily exhibited, because if a thermometer is immersed in the jet the 

 friction of the air gives rise to heat which nearly neutralizes the cold. The tem- 

 perature of one part of a jet may thus be hundreds of degrees different from that of 

 another part. The authors have, in fact, shown that a thermometer may be so 

 placed in a jet as to experience either cold or extreme heat. Hence the absolute 

 necessity in their experiments of a porous plug, which will allow the air to issue in 

 a tranquil flow without jets or rapids. 



A general result they have anived at on transmitting elastic fluids through a 

 porous plug, is, that the thermal effect is proportional to the difference of pressures 

 on the opposite sides. 



A diminution of temperature takes place in all the gases tried except hvdrogen ; 

 and this diminution or cooling effect is decreased when the temperature is raised. 



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