﻿Intelligence and Miscellaneous Articles. 



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E is the common length of the tubes. N may be known for every 

 temperature and under every pressure. For other gases than air 

 one of the tubes was filled with dry air, the other with the gas to 

 be investigated ; the index of this latter was given by the formula 



n'o=^/i"+W-i)+^{n!-i;+^-(i+^).} 



We must know t, H, m ; m is positive or negative according as the 

 gas to be investigated is more or less refracting than air. 



To have a colour of a known wave-length, a pure spectrum was 

 produced, and one of the three lines C, E, G was made to fall on 

 the slit which illuminated the apparatus. 



Second method. — The idea of this method was suggested by the 

 consideration of the formula (1). 



In fact, let us conceive an arrangement of apparatus by which we 

 can measure for each value of X the number of fringes removed corre- 

 sponding to the difference H — H' of the pressures in the two tubes. 



The difference of pressure is measured by a kind of voluminometer ; 

 and while the mercury flows out as the elastic force of the gas in one 

 of the tubes diminishes, the experimenter counts the number of 

 fringes removed. In this method, singularly enough, the compen- 

 sator has but a secondary part, and most frequently its use may- 

 even be suppressed. 



These two methods, which were intended to control each other, 

 have given the same results. It will moreover be understood that 

 the first method admits of numerous means of verification. 



Vapours. — These two methods of experimentation may be applied 

 to vapours. I had intended to apply these methods to all those 

 whose elastic forces are furnished by M. Regnault's Tables. But 

 time and sunlight having failed me, my investigations in this direc- 

 tion could only be extended to bisulphide of carbon, to ether, and 

 chloride of ethyle ; and as the results obtained with these two latter 

 compounds seem to leave something to be desired, I shall not adduce 

 them here. 



Gases. 



1. Air.— Index of white light : 1-0032944. 



Line C N c = 1-0002575. 



Line E N E = 1-0003042. 



Line G N G = 1-0003157. 



The dispersion is therefore 



N G -N E = 0-0000115 ; N G — N c = 0-0000582. 



2. Carbonic Acid. — Index of white light : 1*000449. 



Line C N c = 1*000395. 



Line E N E =1'000456. 



Line G N G = 1 -000426. 



