1020] on Low Temperature Studies 27.") 



vidual absorptions, there still appears some discrepancy, so that 

 probably the index of refraction employed (that for "D" light) is not 

 sufficiently near for the dark radiation employed, especially with a 

 substance of such relatively high index as silver chloride. The 

 observed values may, however, be utilised in a reverse calculation to 

 that given above — i.e. to determine the effective index of refraction 

 to give the observed reflection losses. Using the observed Y = 0*51 

 (49 per cent, transmitted) and m = 6, then y = 0*148, whence 

 n — 2 "2b, instead of the l g 69 employed in the previous table. 



Tabulated Values of Transmissive Power. 



Owing to the great heat-absorptive power of water, the principal 

 precaution to be taken in preparing substances for these observations 

 is to ensure perfect dehydration. 



Among the more simple bodies carbon disulphide and iodine are 

 well known as very perfect transmitters of heat at ordinary tempera- 

 tures, and they preserve this property at low temperatures. Tetra- 

 chloride of carbon and thiophosgene are similarly very transmissive. 

 Phosphorus is remarkable, for while its observed transmission is 

 76 per cent., when this is corrected for reflections less than 10 per 

 cent, remains to represent the maximum absorption, which is thus 

 even less than that of iodine, and of the same order as rock-salt and 

 the chloride and bromide of silver. 



The paraffins and benzene exhibit high transmissive powers, and 

 halogen substitution generally increases the value. Alcohols, ketones, 

 acids and similar bodies are absorptive, as also are amides, nitro- 

 bodies, etc. Esters are generally more transmissive, while pyridine 

 and its associates are still better. 



The principal results are grouped in the following tables. There 

 are four columns- in addition to the one describing the substance. 

 Of these the first indicates the condition of the cooled substance : 

 G- being for a glass and C for a crystalline structure. The support or 

 means of securing the disc is shown in the second column ; 

 RM indicates that the material was solidified in a rubber-membrane 

 tambourine, Oh that the chiffon gauze was employed as already 

 described, and P that the material was in the form of a plate 

 compressed or frozen. The thickness in mm. is given in the third 

 column, and the observed per cent, transmission in the fourth. 



