OF ARTS AND SCIENCES. 131 



able. A comparison was made between tbe thermograph and a ther- 

 mopile of forty-eight couples. Using the same galvanometer for each, 

 the thermograph proved about twelve times as sensitive as the other 

 instrument. The limit of sensitiveness could be very much extended, 

 if it were desired, by the use of a more delicate galvanometer. As 

 actually adjusted, the galvanometer employed gives a deflection of 

 one scale division for 0.00000007 Ampere, the period being ten 

 seconds. The constant of Langley's galvanometer was 0.0000000013 

 Ampere, or fifty times less. 



Radiation from Rocks. 



As experiments upon the radiating power of rocks and minerals 

 were the first in which the new instrument was employed, they will be 

 first discussed. 



The want of a standard of radiation which should be more satis- 

 factory than a lamp-black-coated surface was seriously felt in these 

 experiments. Lamp-black is a hygroscopic substance, and radiates 

 differently when in different states, unless it be applied with sufficient 

 varnish to make it water proof. But if it be mixed with the smallest 

 possible amount of varnish, and applied to a surface, then the radiating 

 power is different from that of the same surface coated by camphor 

 smoke. Also, it appears to radiate differently when applied in the 

 same manner to different substances. The writer has always found 

 ordinary white pumice-stone a better radiator than the same black- 

 ened. There is no other instance in which so variable and imperfect 

 a standard is tolerated. The writer has employed as a working 

 standard in these experiments a piece of pure quartz finely ground 

 with emery but unpolished. Such a standard seems to possess many 

 advantages. Quartz is readily obtainable in a pure state. It is im- 

 perishable and unalterable at any ordinary temperature. It has the 

 great merit of perfect definiteness of composition. 



The measures of radiation from the rocks were made as follows. 

 Each specimen had a wooden handle adapted to it, for convenience in 

 handling when hot. The rocks were heated in an oven intended for 

 drying chemical precipitates. The oven was of heavy iron, and was 

 heated from beneath by numerous small gas jets. It was found that. 

 after the gas had been lighted about an hour, the temperature of the 

 oven became constant, and remained so for any length -.1' time. 

 rock under experiment and the quartz standard were placed in tl"' 

 oven together, side by side, and after the lap- of an hour they were 

 removed, first the quartz and then the rock being quickly taken from 



