in which Kalvrnl Bodies exhilit Colours. 205 



degrees of Reaumur, the thermometer in the chalk indi- 

 cated the temperature of 33 degrees, that in the cimiabar 40, 

 and that in the lamp-black 35. 



It hence follows, therefore, that the conducting power of 

 these substances for heat is as little proportioned as their 

 capacities for it to that heat which the light excites in them: 

 otherwise the lamp-black must have acquired the greatest 

 temperature ; whereas it exhibited the lowest, and conse- 

 quently showed the least power for conducting heat*. 



These results, therefore, are a sufficient proof that the 

 light, when it acts on substances of different colours, is 

 capable of producing different degrees of heat, if in their 

 former state they were free from heat. 



But it is worthy of remark, that the thermometer painted 

 with chalk exhibited a lower temperature than that of the 

 light which fell upon it; and this shows that white light 

 possesses rather tlie property of extracting heat from colour- 

 less bodies than producing in them hcac : which agrees \\ ith 

 some other experiments. 



If the results of these experiments, therefore, be employed 

 to determine whether light be a simple substance, or a pro- 

 duct of the mixture of two different component parts, we 

 are naturally led to adopt the latter idea, namely, that light 

 is not a simple but a compound substance. 



We are thus conducted to the answering of the second 

 question, What are the component parts of light ? We are 

 taught by the above experiments, that chalk, when it comes 

 into contact with a c(jlourless ray of light, not only produces 

 no heat, but even lessens the absolute heat of the light ; 

 and on the other hand, that lamp-black in contact with 

 light is capable of producing the greatest degree of heat. 



The latter is the case also when other black objects are 

 subjected to the action of the light, and is confirmed in a 

 striking manner by the well-known experiment, that snow 

 under a piece of black cloth melts much sooner than under 

 white. 



It hence follows also, and this is agreeable to Newton's 



• Tlie author's infcicnre is in some respects incorrect. The experi- 

 ment was instituted to dutcrniine whether the colours he employed ncttd 

 as conductors of huat, or had different degrees of power to extricate heat 

 from hgiit. The result only proves a certain truth respecting the con- 

 ducting power of the !ubsUi>:cfs eniplnytd. 1 he power of all hodies to 

 transmit heat is. in sotne degree af least, as their decisitits. Their co- 

 lours also may affect the result; hut nuin dissiiril.ir subst.inces we appre- 

 hend no accu.atc inference cjn be diawii respecting the point in qcestion. 

 — EuiT. 



principles, 



