500 PHILOSOPHICAL TRANSACTIONS. [annO l/SQ. 



As, in the experiments above mentioned, heat without light gives colour to 

 the nitrous acid, and the reflection or refraction of light is always attended with 

 heat, it may perhaps be heat universally that is the means of imparting this 

 colour, though the mode of its operation be at present unknown. And in these 

 experiments, as well as the former, it is the vapour that first receives the colour, 

 and imparts it to the liquid when it is sufficiently cold to receive it. The rushing 

 out of a quantity of turbid white air from a transparent tube, quite cold, is a 

 striking phasnomenon in these experiments. It may be worth while to examine 

 of what it is that this remarkable cloudiness of the air consists. There is the 

 same appearance in the rapid production of any kind of air, which is perfectly 

 transparent, as it passes along the glass tube through which it is transmitted, 

 till it comes into contact with the water in which it is received. 



For further observations, we may refer to Dr. P's separate publications on 

 this subject. 



XII. Ohservalions on a Comet. By Wm. Herschel, LL.D., F. R. S, p. 151, 

 December 21, 1 788, about 8 o'clock, I viewed the comet which my sister had 

 a little while before pointed out to me with her small Newtonian sweeper. In 

 my instrument, which was a 10 feet reflector, it had the appearance of a con- 

 siderably bright nebula; of an irregular, round form; very gradually brighter in 

 the middle; and about 5 or 6 minutes in diameter. The situation was low, and 

 not very proper for instruments with high powers. Dec. 22, about half after 5 

 in the morning, I viewed it again, and perceived that it had moved apparently in 

 a direction nearly towards S Lyrae. I had been engaged all night with the 20 feet 

 instrument, so that there had been no leisure to prepare my apparatus for taking 

 the place of the comet ; but in the evening of the same day, I took its situation 

 3 times, as follows: viz. Dec. 22, sidereal time. 



At 23^ 42™ ig^ 23^ 52"^ 52^ . . . O*^ 6"^ 35^ comet passed the wire. 



At 23 49 24 23 59 58 13 40 (3 Lyrse passed it. 



DifF. 00 7 5 00 7 6 07 5 



I found in every observation the small star which accompanies (3 Lyrse, exactly 

 in the parallel of the comet. These transits were taken with a 10 feet reflector; 

 and the difference in right ascension, I should suppose, may be depended on to 

 within a second of time. The determination also of the parallel can hardly err 

 so much as 15^' of a degree. This, and several evenings afterwards, I viewed 

 the comet again with such powers as its diluted light would permit, but could not 

 perceive any sort of nucleus, which, had it been a single second in diameter, I 

 think, could not well have escaped me. This circumstance seems to be of some 

 consequence to those who turn their thoughts on the investigation of the nature 

 of comets; especially as I have also formerly n;ade the same remark on one of 



