207 



author expressed the opinion, that so long as new forms are daily 

 discovered (and that this is the case he proved by many recent ex- 

 amples), we are liable to err in establishing both genera and species. 

 He therefore recommended the collection and figuring of all such 

 forms as appear distinct, to which, of course, provisional names must 

 be given, with a view to the future employment of these materials, 

 when new forms shall have become rare, in ascertaining the true 

 natural groups, whether generic or specific. 



The author took occasion, from the occurrence of the permanence 

 of characters above alluded to in many species, to combat the view 

 of Professor Kiitzing, according to whom, species, as natural groups, 

 do not exist. 



Finally, he stated, that the remaining forms would be described 

 in a future communication. 



2. On the Physical Appearance of the Comet 3, of 1853. 

 By Prof. C. Piazzi Smyth. 



Referring to the general descriptions which had been published 

 in scientific journals and elsewhere of the appearance of this comet, 

 the author pointed out, — 1st, That the colour which had been attri- 

 buted to it was merely the adventitious tint due to the twilight at- 

 mosphere through which it was seen. 2dli/f That what had been 

 described as the nucleus of the comet, and of so many thousand miles 

 in diameter, nine days before the perihelion passage, was merely the 

 head, composed of the same light, vaporous transparent matter as the 

 tail ; and subject to the same remarkable compression and conden- 

 sation on approaching the sun. 



This condensation had not been sufficiently attended to by comet- 

 ary observers ; but, nevertheless, rendered it absolutely necessary, in 

 giving the size of any comet, to state at v^hat part of its orbit the 

 body might be at the time. The now well recognized fact of such 

 condensation, combined, of course, with the stronger illumination 

 of the sun at a less distance, also gave the best, if not the only, suf- 

 ficient explanation of the remarkable increase in brightness of some 

 comets about the time of their perihelia. 



Moreover, the accurate observation of the amount of such con- 

 densation, depending as it does mainly on the proportion between 

 the aphelion and perihelion distances, might lead in many cases to 



