390 Royal Society. 



and one or two other salts*. On the other hand, fine weather has 

 its advantages ; not only are the surfaces of the solutions more 

 active and the evaporative force stronger, but hydrated salts, said 

 to exist in the air, part with their water of crystallization so readily 

 as to reduce them to the non-nuclear condition. This is especially 

 the case with sodic sulphate. I have already shown that super- 

 saturated solutions of this salt may be exposed to the air, both in 

 fine and wet weather, for a long time without crystallizing t,*as 

 well as the fact just noticed %, that a solution of a salt does not 

 act as a nucleus to its supersaturated solution. Moreover, if sodic 

 sulphate exist in the dusty air of a room, it cannot retain the hydrated 

 form during many minutes, but must rapidly pass into the anhy- 

 drous, in which it is no longer a nucleus. 



Seeing, then, that in the case of sodic sulphate, which is said 

 to be always present in the air of rooms, and, according to MM. 

 Gernez and Viollette, even in that of the country, the chances are 

 that it is most likely to be present either in the effloresced condition 

 or in solution, and equally non-nuclear in both, I cannot help 

 hinking that too much importance has been given to this part of 

 the subject; for if it be true, we are reduced to the dilemma, 

 pointed out by M. Jeannel §, that there must be floating in the air 

 specimens of all kinds of salts that form supersaturated solutions 

 and crystallize by the introduction of a solid nucleus, whereas 

 there are some such salts which cannot exist in the presence of the 

 oxygen or of the ammonia of the air. 



March 27. — Sir George Biddell Airy, K.C.B., President, in the 



Chair. 



The Bakerian Lecture. — " On the Badiation of Heat from the 

 Moon, the Law of its Absorption by our Atmosphere, and its va- 

 riation in amount with her Phases." By the Earl of Bosse, D.C.L., 

 F.R.S., &c. 



In this paper is given an account of a series of observations made 

 in the Observatory of Birr Castle, in further prosecution of a 

 shorter and less carefully conducted investigation, as regards 

 many details, which forms the subject of two former communica- 

 tions || to the Boyal Society. 



The observations were first corrected for change of the moon's 

 distance from the place of observation and change of phase during 

 the continuance of each night's work ; and thus a curve, whose 

 ordinates represented the scale-readings (corrected) and whose 

 abscissae represented the corresponding altitudes, was obtained for 

 each night's work. By combining all these, a single curve and table 

 for reducing all the observations to the same zenith-distance was 

 obtained, which proved to be nearly, but not quite, the same as 



* Chemical News, February 4, 1870, p. 52. 



t Proceedings of the Boyal Society, 1871, p. 41. 



X Chemical News, February 4, 1870, p. 52. 



§ Ann. de Ch. et de Ph. 4th ser. vol. vi. p. 166. 



|| Proceedings of the Koyal Society, vol. xvii. p. 436, vol. xix. p. 9. 



