AIR BENEATH THE MICROSCOPE. 
173 
and the prevalence of any of these diseases. 12. The amount 
of inorganic and amorphous particles and other debris suspended 
in the atmosphere is directly dependent on conditions of mois- 
ture and of velocity of wind. 
Dr. Cunningham concludes his observations with some valu- 
able suggestions as to the reasons why his inquiries differ from 
those of Ehrenberg. For example, he shows that the methods 
employed by the two observers were totally distinct. The 
German worker obtained nearly all his specimens, not directly 
from the air, but from leaves, pieces of bark, bits of moss, 
&c. But of course this is a most unreliable mode of ope- 
rating; for when surfaces have been wetted with rain, there 
is no difficulty for rotifers, tardigrades, and so forth, to make 
their way over them to a considerable extent ; and it is this 
circumstance, doubtless, which gave the foreign observer such 
a wide zoological range to his collection. Dr. Cunningham’s 
mode could alone gather what was actually in the air ; hence 
his series of specimens have not been so wonderfully strange. 
The author of the last work on the subject suggests in his 
final remarks that these collections might, if kept and then 
studied, have formed a different tale for the observing student. 
And here, doubtless, he has hit upon the right cord. We 
greatly regret that the idea did not occur to him before, as 
then it might have enabled him to give us something more 
worthy of a genuine microscopical student. It is, then, in 
absence of this and of another fact that, it appears to us, the 
author has, through omission, committed a serious error. 
Two questions arise from the series of researches which he 
and others have conducted, and which have been described in 
the earlier pages ; and these are — 1. Are there always in the 
atmosphere a quantity of animal and vegetable germs ready, at 
any moment when offered the favourable conditions of develop- 
ment, to come into existence ? 2. Do a certain number of these 
(animal or vegetable) promote those various epidemics and 
other diseases, so terrible in their manifestations, and which 
come and go in many instances just as a cloud of minute par- 
ticles might be driven by the wind ? The first of these ques- 
tions is, it seems to us, partly answered in the affirmative ; and 
it only requires the employment of the highest powers and the 
necessity of most careful watchmanship on the part of the 
student in order to have a complete and decisive reply. The 
Bev. Mr. Dallinger, F.K.M.S., has shown us by his recent 
inquiries (“ Monthly Microscopical Journal,” January, Febru- 
ary, and March, 1874) how a whole host of organisms may 
completely escape observation by the use of powers even 
so high as those Dr. Cunningham has employed. It is only, 
then, by the employment of objectives of the -/j-inch and 
