404 M. H. Becquerel's Experimental Investigations 



consequence of the passage of the electric current through 

 the coils, the temperature of the tube rose rapidly to 30° or 

 40° C. It was therefore very important to ascertain the 

 average temperature of the gases in each series of experi- 

 ments. 



To do this I measured the temperature of the copper by 

 means of contact-thermometers. Moreover the tube itself 

 acted as a gas-thermometer ; it was in constant connexion 

 with a mercurial manometer which, when the atmospheric 

 pressure was ascertained, gave at every moment the pres- 

 sure of the gas inside the tube. The temperature deduced 

 from observations of the pressure was always the same as 

 that given by the thermometers. The comparison of these 

 temperatures enabled us to discover when there were leakages 

 of gas in the tube. 



In order to admit of comparison, the results observed had 

 to be reduced to the same pressure of 760 millim. of mercury 

 and to the same density. The correction relative to the pres- 

 sure and to the density might amount to 0*10 of the value of 

 the numbers found experimentally. 



I have assumed that the magnetic rotatory power of the 

 gases studied between 0° and 40° varied proportionately to the 

 density, or in the inverse ratio of the modulus of dilatation 

 (1 + at). The probable existence of a slight variation with 

 the temperature in the molecular rotatory power of the gas 

 has been thus neglected ; bat it may be remarked that if this 

 variation is of the same order of dimension for gases as for 

 other substances it must escape our measurements; for it is of 

 the order of the errors of observation. The agreement of the 

 results obtained at different temperatures seems to justify this 

 hypothesis. Concerning the numbers given below, it may be 

 stated that they were obtained at a temperature of between 

 10° and 40°, and that the different gases were reduced to a 

 uniform pressure and volume by multiplying the numbers 



obtained by the factor -^- (1 + at), H being the pressure of 



the gases, t their temperature, and a their coefficient of dila- 

 tation. 



V. Displacement of the Luminous Point. — The luminous 

 point does not invariably occupy a fixed position. Although 

 tie blowpipe is fixed to the polariscope, it happens that the 

 luminous point rises or falls on the piece of lime according 

 to the greater or less pressure of the oxygen which feeds the 

 combustion. The lime itself becomes slightly hollow; and on 

 this account the incandescent region is displaced. 



These slight displacements of the luminous point cause 



