776 PHILOSOPHICAL TRANSACTIONS. '[ANNO 1800. 



proved by the Q4th experiment, that our bluish-white glass stops a considerable 

 portion of the heat that goes with the red rays; then, if the 86 rays of light 

 which this glass stops, are supposed to be all of that sort, the heat which will be 

 stopped in consequence will, according to the experiment we have mentioned, 

 amount to 86 multiplied by .375, that is, 32 rays of heat; but since 250 have 

 been stopped, there will remain 218 to be accounted for. In this calculation, a 

 manifest concession has been made, which ought to be explained. When I men- 

 tion 86 red-coloured or red-making rays, I mean so many of them as will make up 

 , g g o of the whole effect of light ; for the quantity of heat and light transmitted, 

 or stopped, in all the experiments that have been given, has been reduced to what 

 proportion it bears to unity ; and having afterwards represented the joint effect of 

 every ray of heat and light by 1000, each mean ray of heat must be the 1000th part 

 of that effect ; but a mean ray of light, though it be likewise the 1000th part of 

 the whole effect of light, will not be so of heat, because the whole effect of the latter 

 is partly owing to rays that have been proved to be invisible. On this account, the 

 86 mean rays of red light, stopped by our bluish-white glass, cannot even amount 

 to a stoppage of 32 rays of heat, which we have allowed. 



As I have made the concession on one hand, I must explain an advantage that 

 may be claimed on the other ; which is, that mean rays and promiscuous ones 

 have already, in a former paper, been proved to differ considerably, and that it 

 remains therefore unknown how many red-making rays we may suppose to be 

 stopped, in order to make up 86 mean rays of light. In answer to this however, 

 I must observe, that the number of promiscuous rays of light and of heat must 

 always be inversely as their power of occasioning those sensations ; so that if, for 

 instance, a red ray is supposed to be twice as heating as a green one, there will 

 only go half the number of them to make up a certain effect of heat; and, on 

 the other hand, if a green ray should have a double power of illuminating, there 

 will be no more than half the number of them necessary to occasion a certain 

 effect of light. But, by my former experiments, a red ray, though much inferior 

 to a green one, is probably fully equal in illumination to a mean ray of all the 

 colours united together. Now as red rays hare also been proved to be accom- 

 panied by the greatest heat, and as our bluish-white glass stops hardly any invisible 

 heat rays, we have certainly gone the full length of fair concessions, by allowing 

 all the light stopped by this glass to be of that sort ; and thus it seems to be 

 evident, that the heat which lies under the colours, if I may use this expression, 

 may be stopped, without stopping the colours themselves. 



It will not be necessary to lay much stress on this single experiment ; our 2d 

 table affords us sufficient ground on which to rest our more forcible arguments. 

 A dark-red glass, for instance, was found to stop 606 rays of heat, and 999-8 of 

 light. This, even at the very first view, seems to amount to a total separation 

 of the two principles ; but let us discuss the phenomenon with precision. As 

 only one ray in 5000 can make its way through this glass, it is evident, that if 



