EEYIEWS. 
77 
and in this he included the exotic as well as the native genera. His reason 
for combining the two in the one classification was a belief that some of 
the species of exotic sponges would before long be found among our British 
sponges. Strange to say, this idea of the author’s has since been realised $ 
for he has himself, since the publication of his first volume, added two 
species of Ecionemici and one of Opli iltasp on gia, an entirely new genus, to our 
list of British species. Dr. Bowerbank offers some useful remarks on the 
identification of sponges ; generally speaking, he says there is no great 
difficulty in the determination of the genus, u but in some cases more than 
ordinary caution is necessary in the examination of the specimen under 
consideration.” He especially cautions the student against the examination of 
sponges by the naked eye alone, for it happens that in many cases the author 
has been unable to make a first identification until he had submitted the 
structure to minute microscopical examination. Dr. Bowerbank’s second 
volume completes his first one, and the two parts form a work which is 
creditable to its author and to the useful society which has published it. 
WELLS ON DEW.* 
F EW of our readers can have failed to have seen reference to Wells’ 
Essay on Dew, so much eulogised as a splendid example of experimental 
research and philosophical induction. Mr. Casella has done good service to 
the scientific public in putting again within their reach copies of this esteemed 
work. Wells’ theory of the natural formation of dew, and since his time 
the one universally received, is that it results from a reduction of temperature 
taking place in the bodies upon which the dew forms, by which they become 
colder, to a sufficient extent, than the atmosphere around them. Previous to 
the publication of this theory, the coldness of bodies on which dew was formed 
was held to be an effect of the dew instead of its cause. The reduction of 
temperature which causes the dew is the consequence of the radiation of heat 
from bodies. Dew is formed at (and not let fall upon) the surface of bodies, 
from the aqueous vapours in the atmosphere u in a pellucid state.” This 
occurs when the air is cooled, by contact with the cold bodies, to that point 
at which the moisture in the air is sufficient to charge it to repletion. 
This theory explains, among other observed facts, why different quantities 
of dew are formed upon bodies of the same kind in different situations ; why 
dew forms on clear nights, and not on cloudy nights ; why the same degree 
of cold in a body may be attended with the formation of much, little, or no 
dew ,* why dew forms on some substances more than on others ; why dew 
and frost (frozen dew) deposits on the projecting parts of bodies more 
than on the massive parts ; and, indeed, why all the varied phenomena of 
the formation of dew and frost occur as they do. 
* “ An Essay on Dew, and several Appearances connected with it.” By 
William Charles Wells. Edited, with Annotations, by L. P. Casella, 
F.R.A.S. ; and an Appendix by It. Strachan, F.M.S. London : Longmans. 
1866. 
