1875.] Notices of Books. 541 
for adulterating white-lead. Mineral oils are widely diffused, but 
nowhere occur in quantity. The saccharoid sandstone, which is 
100 feet thick at Cape Girardeau and 133 feet at St. Charles and 
Warren, is an excellent material for the glass manufacture. 
Electricity ; its Theory, Sources, and Applications. By Joun T. 
SpraGuE, Member of the Society of Telegraph Engineers. 
Peudon =. 5..and PN. Spon. 1875. 
Tue Author having contributed to the ‘“‘ English Mechanic” a 
number of articles on Electricity, now collects them for the first 
time; and his object in making this compilation has been to review 
and systematise the leading facts of the science, and thus to pro- 
vide for general readers, interested in the subject, a catalogue rat- 
sonnée. The work is divided into twelve chapters, followed by a 
short ‘‘ Dictionary of Terms.” The first chapter is introductory, 
and at the outset our knowledge is divided into Metaphysics and 
Physics, the latter being designated ‘‘ the basis of all our real 
knowledge.” Various definitions follow :—Matter, elements, 
atoms, atomicity, molecules, chemical notation, ether. These 
definitions are slight, and sometimes very incomplete, and even 
inaccurate. ‘Thus we are told that ‘‘ the atom has several rela- 
tions to force, while gravity, heat, and chemical affinity are the 
only forces specified.” Again, of the ether it is said that astro- 
nomy affords us actual evidence of its existence, yet that we 
know “ absolutely nothing” about it. Although, indeed, astro- 
nomy furnishes one proof of its existence, it is, we think, by no 
means the principal proof, and no one who has studied the phe- 
nomena of polarised light can say that we know absolutely 
nothing of the ether. In discussing valency, we are told that 
‘‘one of the oldest ideas of the atom was that matter had no 
real existence, and that atoms were simply centres of force ;” 
but surely this was one of the later phases in the history of the 
atomic theory. The division of molecules into ‘‘ molecules 
which are also atoms,” and ‘‘ molecules formed of two similar 
atoms,” is singularly perplexing to the student, and surely a 
very novel idea. Finally, we have a classification of the forces, 
and an expression of the belief that electricity, like heat, is a 
mode of motion. Altogether we think this introductory chapter 
will be of little use to the general reader unless he has already 
thought over for himself the various subjects which it discusses, 
and can thus discriminate between hypotheses and facts, and at 
the same time supplement the somewhat partial knowledge 
which it conveys. The second chapter, which occupies about 
one-seventh of the book, treats of static electricity. This sub- 
ject is treated in a somewhat abstruse and occasionally very 
hypothetical form: for example (p. 22) it is pure hypothesis to 
assume that the molecules of bodies are spherical, and that if 
