300 Intelligence and Miscellaneous Articles. 



After various trials, the interference method employed by 3VX, 

 Jamin, and described in his classical memoirs on the variation of 

 the indices of air, water, and aqueous vapour, was selected by us 

 as being the most precise ; but it left us confronted by a difficulty 

 which it had not had to surmount : the apparatus employed in 

 previous experiments had only to support pressures, at the most, 

 equal to the atmospheric pressure, while we wished to reach 60 or 

 70 atmospheres. We decided the question by using an apparatus 

 already described and employed by one of us*, and which a modi- 

 fication in certain details sufficed to render suitable for the sort of 

 experiments we wished to attempt. 



This apparatus, constructed by M. Grolaz, permits the compres- 

 sion of the gas under examination to take place in a prismatic 

 cavity pierced in the interior of a block of steel of 20 centim. 

 length and closed at each end by a glass plate of 1 centim. thick- 

 ness, firmly fixed. One of the interfering pencils furnished by M. 

 Jamin's first mirror traverses this cavity ; the other pursues, in free 

 air, a parallel path at a distance of 1 centim. Two glass plates 

 identical with the first were interposed in the path of this second 

 pencil. The two pencils afterwards pass through a compensator, 

 and are received upon the second mirror, where they interfere. 

 The fringes are observed horizontal and directed into a telescope 

 furnished with a reticule. 



In a first series of experiments we were able to follow the cen- 

 tral fringe (white light) up to 65 atmospheres. Desiring to reserve 

 to ourselves the time necessary for making these measurements 

 with all the care which the} r require, we shall at present give only 

 the result of measurements made between 24 and 36 atmospheres. 

 In the third column of the annexed table will be found the number 

 of the fringes (yellow \ of sodium) which pass under the reticule 

 of the telescope for a variation of pressure given by the difference 

 of the numbers in the first and second columns. Temperature 22°. 

 Pressure. Number of 



Initial. Final. fringes. n. 



24-5 atm. 28-5 atm. 335 0-550 



28-5 „ 32-5 „ 311 0-510 



32-5 „ 36-5 „ 338 0-555 



"We have calculated the number n of the fringes which would 

 pass under the reticule for a variation of pressure of 1 milhm. of 

 mercury in a tube of 1 metre length ; the results of the calculation 

 are indicated in the fourth column. 



The pressures were estimated with the aid of a metallic mano- 

 meter which, unfortunately, had not sufficient precision to permit 

 more regular results to be obtained. Nevertheless the numbers in 

 the fourth column differ but little from the number 0-556, which is 

 calculated on adopting for the index of air at 22° the value 0*000271, 

 found by M. Jamin at the atmospheric pressure. — Comptes Rendus 

 de V Academie des Sciences, March 12, 1883, t. xcvi. pp. 699-701. 



* J. Chappuis, "Etude spectroscopique sur l'ozone," Annates Scien- 

 tifiques de VEcote Normale superieure, 2 e serie, t. xi. April 1882. 



