238 Intelligence and Miscellaneous Articles. 



This reaction must then increase with the value of a 1 , and attain its 

 maximum when the circuit is open. 



These laws being recognized and verified, we investigated the 

 only application yet made of these machines, the electric light. 

 Whenever a regulator is interposed in the path of the current, the 

 velocity of the machine is diminished, as it would be by the interpo- 

 sition of a metal wire. The arc exerts a resistance co which can he 

 determined by ascertaining how many turns of a rheostat must be 

 introduced into the circuit to produce an equal retardation in the 

 velocity of the machine. That being done, we have compared the 

 heat disengaged by this are with that reproduced in the resistance, and 

 we have found that both are exactly equal. We have then been led 

 to the conclusion that the arc only acts as a metal wire, either by 

 the heat it regenerates, or by the diminution it produces in the in- 

 tensity of the current. 



This heat of the arc is very weak, scarcely equal to that produced 

 by a gas jet burning a litre in a minute. To obtain this result, 100 

 litres of gas were consumed in Ilugon's motor ; and the heat repro- 

 duced docs not exceed the hundredth of that consumed. But 

 though small, as it is concentrated on a very narrow space on 

 charcoal-points, it developcs there an enormous temperature, and 

 an intensity of light almost twice as [much as would be obtained 

 by directly burning the 100 litres of gas expended in its produc- 

 tion, and four times as much, even, if the carbons are prepared by 

 Carry's process. 



There is thus an enormous loss of heat on the one hand, a re- 

 markable gain of light on the other. 



There is nothing paradoxical in this result. The magneto-elec- 

 trical machine only utilizes a small portion of the heat absorbed ; 

 but it collects it disseminated over a large space, and concentrates 

 it in a small volume ; it takes it at a low temperature to pro- 

 duce an enormous heating of the carbons ; it finds it as obscure 

 heat to change it into light ; it diminishes its quantity, and trans- 

 forms its nature ; it expends calorific radiations which are cheap, 

 and makes them into luminous radiations which are dear ; and it 

 finally gives them at a cheaper rate than any other source of 

 light. — Complex Rendu s, June 1, 1868. 



ON THE EXPANSION AND COMPRESSION OF SATURATED VAPOURS, 

 BY M. CAZIN. 



I beg to communicate the result of researches on saturated vapours 

 on which I have been engaged for the last two years. The question 

 was this — to observe under what circumstances a vapour is partially 

 condensed when its volume is changed without either addition or 

 loss of heat. 



At the sitting of the Academy, January "25, 18GG, I communicated 

 a first series of experiments, on expansion alone. Since then I have 

 constructed an apparatus by which the expansion or compression of 

 vapours at high temperatures may be effected at pleasure. 



A horizontal copper cylinder, closed by parallel glass plates and 

 heated by the aid of an oil-bath, contains vapour in a state of satu- 



