98 The Microscopical Examination (yous Ae aeto. 
before its birth. Now, before the wort is made, and the yeast- 
plant developed, molecular change has really already begun. Its 
effect is the difference between malt and raw grain. Starch has 
been altered into dextrine and sugar without the yeast-plant, which 
may next assist by its growth or decay (as some think) to continue 
the alteration until alcohol and vinegar result. In castor-oil and 
colza seeds, Frémy found that the fatty matter was converted into 
dextrine and sugar by fixation of oxygen, without, of course, the 
assistance of the yeast-plant. Payen and Persoz attributed such 
change to the action of diastase ; later observers assert that it can be 
effected, under suitable circumstances, by any albumoids, they, them- 
selves, being partially altered by the action of oxygen at a certain 
temperature, with moisture present. Now, in the mucus present 
in the atmosphere, as shown in my previous paper—in those gra- 
nules resembling exudation granules, found plentifully in the at- 
mosphere of a fever patient, and whose contagious influence was 
mentioned—in these, and such as these, the requisite albumoid 
substance is found. And I am convinced that it depends upon 
whether or not these or such albumoids be in a condition of motor- 
change, to determine whether or not we shall have present that 
contagious matter whereby certain diseases are engendered and 
communicated. 
VII. —The Microscopical Kxamination of Rocks and Minerals. 
By 8. Atzrort, F.G.S. 
Tue application of the microscope to Geology is a subject which 
has, I believe, been more generally neglected by men of science 
than any other of equal importance ; although it is easy to show 
that this method of examination affords the readiest means of 
obtaining information on many important points, and is, in fact, 
the only way of ascertaining with certainty the composition of 
many compact or fine-grained rocks. 
A valuable paper by Mr. Sorby, “ On the Microscopical Structure 
of Crystals,”* and another by Mr. David Forbes, “On the Micro- 
scope in Geology,” in the ‘ Popular Science Review,’ { constitute, I 
believe, the extent of the published information on the subject. No 
branch of science presents a more promising field of inquiry, for the 
nomenclature and classification of igneous rocks is in a lamentable 
state of confusion, and unfortunately the maps of the Geological 
Survey of England and Wales tend rather to increase the difficulty 
by the indiscriminate use of terms not very definite in themselves. 
It is evident that no satisfactory classification of rocks can be 
* ‘Jour. Geol. Soc.,’ vol. xiv. + Vol. vi. p. 355. 
