Some Physical Pityicrtics of Nitric Acid Solutions. Ill 



required, it would be sufe to make all the necessary observations and 

 readings. 



Our experience has been that if by any chance the prism-contents 

 were altering in temperature the edges of the D-line appeared some- 

 what blurred, but if the temperature was constant the line was per- 

 fectly sharp and well defined. Though this method of working is 

 accompanied by some personal discomfort, yet it is believed to be 

 susceptible of greater accuracy than when only the temperature of the 

 prism is varied. 



VIII. Refractive Index of 



As a preliminary experiment determinations were made of /* for 

 water, purified by the methods adopted by us ; the values found were 

 pjw == 1-33376 and /V s = 1 '33328; the latter does not differ 

 widely from the mean value /i D 20 = 1 '33334 obtained by former 

 observers, whose results have been recently collated by Rudolphi.* 



IX. Method of Observation. 



The method of conducting a determination was as follows : 



(i) The prism was washed out four times with portions of the pre- 

 pared sample of acid, then finally charged, allowed to stand 

 for a short time and the thermometer inserted ; 



(ii) Spectrometer telescope adjusted for viewing the collimator slit 

 directly, the point of intersection of the cross lines being made 

 to coincide with the left-hand edge of the image of the slit, 

 and a reading taken ; 



(iii) Charged prism placed upon the table and the position of 

 minimum deviation for the D-line found, the temperature being 

 noted ; 



(iv) Prism removed and (ii) repeated ; 



(v) Kepetition of (iii) after a short interval. After several hours a 

 second set of experiments was made. 



The following are the details of one particular set of de- 

 terminations : 



Date, 12.4.1900. Concentration of acid = 71 '57 per cent. 

 First set, temp. 15 C< 3 (corr.). Hour, 2.50 P.M. 



Micrometer No. I. Micrometer No. II. 



,. . Direct readings ......... 180 0' G" 360 0' 8" 



Deviation 150 36 4 330 35 56 



Difference = Min. Dev. 29 24 2 29 24 12 



Mean... 29 24' 7". 

 * 'Zeits. Physikal. Cbem.,' vol. 37 (1901), p. 433. 



