Notices of Books . 
hi 
1876.] 
genera, and even groups of higher rank, are sometimes 
totally overlooked. The theoretical articles, see e.g. “ Species,” 
are characterised by a certain feebleness and looseness of 
thought. 
Chemistry and mineralogy are treated with greater ability, or 
at least with more care, whilst the articles on astronomy and 
mathematics will, we think, exceed both the wants and the com- 
prehension of all readers except such as have made these sciences 
the subjeCt of special study. 
Having thus pointed out what from our point of view appear 
as imperfections, we have pleasure in adding that the work con- 
tains a vast amount of useful and valuable information. He 
must be indeed a learned man who in turning over these 
volumes does not occasionally come upon some faCt of which 
he was before totally ignorant. This, indeed, is the chief use of 
encyclopaedias. They are not intended to make specialists, but 
to supply that general knowledge without which the most accom- 
plished specialist often finds himself “ at sea,” and which he has 
not the time nor the opportunity to seek out in its original 
sources. 
Air and its Relations to Life. By Walter Noel Hartley, 
F.C.S. London : Longmans, Green, and Co. 
This book is, as the title-page informs us, the substance of a 
course of leCtures delivered at the Royal Institution in the sum- 
mer of 1874. The character of the work is avowedly “ light 
and popular,” as its origin necessitates. Still, though scientific 
technicalities have been as far as possible avoided, there is no 
room to complain of a want of accuracy. The work is more 
comprehensive than might perhaps be anticipated from its title. 
The author treats successively of the proof of the existence of 
the air, of Priestley’s discovery of oxygen, and Lavoisier’s dis- 
covery of the nature of the air ; of the reasons for regarding the 
air not as a true chemical compound, but as a mere mechanical 
mixture, and of Tessie du Motay’s process for obtaining oxygen 
from the air. In the second chapter, Mr. Hartley treats of Black’s 
experiments on the carbonating of lime ; on the properties of 
carbonic acid ; the presence of aqueous vapour and of ammonia 
in the atmosphere ; the preparation of ozone ; on Dr. Angus 
Smith’s method of determining the proportion of carbonic acid 
in the air ; and of the reciprocal adtion of plants and animals. In 
the next chapter, we find an account of the means whereby a 
constancy in the composition of the atmosphere is maintained. 
This leads to a consideration of the expansion of gases by heat, 
the intermixture of gases in apparent opposition to their specific 
gravities, and on gaseous diffusion. This naturally leads up to 
the subjedt of ventilation, with the evil effedts of foul air, and a 
