May 19, 1892] 



NATURE 



53 



While thus giving to the author warm praise and 

 congratulation, we cannot avoid noticing serious faults 

 both of commission and omission. First it seems simply 

 deplorable to drag quaternion notions and notation into 

 an elementary book of this kind, unless it be to show how 

 ridiculous the riders of the quaternion hobby can at 

 times become. The explanations and definitions at the 

 commencement of chapter v. will be nothing to the 

 majority of learners and teachers but a mass of confusion 

 thrown over one of the simplest and most important of 

 subjects. To prove by quaternions the formula S = VT 

 (space described in a given time with constant velocity), 

 which needs only a knowledge of the multiplication table ; 

 or the formula .v = hgt- for falling bodies, which can be 

 explained by common-sense (but not by quaternions) to 

 a boy of twelve in half an hour, is simply inexcusable. 

 Wherever quaternions are introduced in this book we 

 find an easy matter made difficult — if not, as in the case 

 of simple harmonic motion, absolutely unintelligible. 

 Unfortunately, Dr. Peddie is not the first writer who has 

 <:ontrived, by means of quaternions, to make a subject 

 unnecessarily difficult and repulsive. 



But by far the most serious defect of this book, and it 

 is one which will greatly mar both its usefulness as a 

 text-book and also its popularity as a somewhat ele- 

 mentary work for reading and consultation, arises from 

 the failure of its author to catch, even in a remote degree, 

 the spirit which has animated and directed the whole of 

 the best experimenting in physics for the last twenty-five 

 or thirty years. Et ignein regunt numeri is the motto of 

 Fourier's great work ; and a realization of the fact that 

 numbers (not merely numerical ratios) must be sought 

 for as the crown of physical laws is that which has given 

 pre-eminent value to the labours of experimenters during 

 the last half-century, and has forced workers in this great 

 field into precision and definiteness. The example set 

 by Gauss and Weber, Joule and Thomson, and by the 

 British Association Committee on Standards of Elec- 

 trical Resistance appointed in 1861, has revolutionized 

 ideas as to what is the ultimate object of experiment- 

 ing in physics ; and we can no longer be satisfied with 

 knowledge as to almost any physical phenomenon until 

 we are able to apply to the phenomenon and to our laws 

 the searching test of arithmetical calculation in absolute 

 numbers. 



Unfortunately, in the book before us there is no recog- 

 nition of these necessary conditions for completeness of 

 knowledge, and very little recognition of recent investiga- 

 tions of the kind here indicated. The failure will be 

 felt most seriously in the important subjects of heat, 

 magnetism, and electricity. 



In electricity there is not to be found the resistance, 

 whether in ohms or in C.G.S. units, of any wire of any 

 material ! There are pages of algebra on dimensions of 

 units, to puzzle the unfortunate learner, but nowhere can 

 he find what an ohm, or ampere, or volt is : unless, 

 " ohm = icP C.G.S." ca.n betaken as a definition, when the 

 meaning of a C.G.S. unit is not explained. Faraday's laws 

 of electrolysis, got fifty years ago, are stated ; but the deter- 

 minations of Lord Rayleighand Kohlrausch of the amount 

 of silver deposited by an ampere current in a second are not 

 even referred to. Tait's thermo-electric curves, and some 

 forms of galvanic cells are described ; but how to find the 

 NO. I 177, VOL. 46] 



electromotive force of any one combination in volts is not 

 indicated. We must not multiply instances. It would be 

 only wearisome. Magnetism, electro-dynamics, are 

 treated in precisely the same way ; and the student would 

 find it impossible to calculate from data in this book 

 how much heat is conducted across a stone slab in an 

 hour under given conditions, or how much heat is lost 

 from the surface of a sooted globe in a minute, though 

 there is a great deal of exposition of laws of heat ex- 

 changes, and of the algebra pertaining thereto. Diff^usion 

 of matter is another subject which suffers from defective 

 treatment in a similar way. The word " diffusivity," intro- 

 duced by Thomson, is correctly defined on p. 131 ; but 

 ten lines lower down the definition is departed from, and 

 a column of relative numbers is substituted for the now 

 fairly known absolute diffusivities. A very thorough 

 change of all these parts of the book ought to be made in 

 a reprint or new edition, in order to make the work con- 

 formable to modern knowledge and requirements. 



It would be ungracious to point out too many minor 

 faults in a first edition ; but a few must be mentioned. 

 Faraday seems to have been forgotten in connection with 

 liquefaction of gases I and Melloni, though not perhaps 

 absolutely trustworthy, surely deserved to have his name 

 mentioned in connection with radiation of heat. Mayer's 

 name is not mentioned ; and, whatever Dr. Peddie may 

 think on the subject of the celebrated controversy, 

 no one will agree with him that the name should 

 be omitted. We cannot help feeling that there is too 

 much local colouring about many parts of the book. A 

 book of this kind is sadly marred by want of proportion- 

 ate distribution of treatment ; even the occupation of 

 space with minute treatment of a favourite subject 

 becomes an injustice with regard to those subjects which 

 are unduly curtailed for want of more space. We trust 

 it will not hurt the feelings of anyone if we remark that 

 the book should be a little more cosmopolitan, and a good 

 deal less Scotch. 



On p. 337, there is a mistake which will bear compari- 

 son with Lord Brougham's celebrated idea that people 

 carry weights on their heads to have them farther from 

 the centre of the earth, and therefore less attracted. The 

 formation of ice in " very hot " countries on shallow 

 pools is compared with Faraday's experiment of freezing 

 mercury in a white-hot crucible. It is radiation, not 

 forced evaporation, which is the cause of the phenome- 

 non referred to. 



We regret, also, that Dr. Peddie has thought it advis- 

 able to follow the example of Maxwell and others in 

 changing Andrews's diagram right for left. There is no 

 reason for doing so. The diagram was much better as 

 Andrews originally gave it ; and it would be better also 

 without the dotted line said to separate the region in 

 which liquid and gas can exist together from the regions 

 in which the substance is entirely liquid or entirely 

 gaseous. The former is a region concerning which there 

 has been much speculation of an unprofitable sort. The 

 elementary student need not be troubled with it, and it 

 cannot be explained to him in a single sentence. 



On p. 95 there is a diagram of a cord being pulled 

 through a tube. Perhaps it cannot be asserted that the 

 diagram is absolutely wrong, because the cord is said in 

 the text to be perfectly flexible. But the cord, passing 



