194 



NA TURE 



[December 28, 1893 



of a very simple character, must suffice. Thus in the 

 strain of a homogeneous isotropic solid, due to external 

 potential u, we have for the strain-function </) (when there 

 are no molecular couples) the equation 



S.V(pa + SaV« = O, 



which (in virtue of the property of a, already spoken of) 

 is equivalent to three independent scalar conditions. 

 !j'ippose we wish to express these, without the a, in the 

 form of one vector condition. Mr. McAulay boldly writes 

 the first term as 



S.o^i'Vj ; or rather as S.a</)V, 

 for in so simple a case the suffixes are not required, and 

 the strain-function is self-conjugate under the restriction 

 above. Then, at once, the property of a shows us that 



<l>v + Vti = o, 

 which is the vector equation required. Here it is obvious 

 that, in the usual order of writing, 



4)V = — ^i + --<pj + ~<pk. 

 ax ay dz 



This simple example shows the nature of the gain which 

 Mr. McAulay's method secures. Those who wish to know 

 its extent must read the work itself. They will soon be 

 introduced to novel forms of concentrated operators with 

 regard to which, as I have not yet formed a very definite 

 opinion, I shall content myself by hazarding the remark 

 that, while they are certainly powerful and eminently 

 useful, they must at present be regarded as singularly 

 uncouth. 



As a purely personal matter I would add that I do not 

 think Mr. McAulay states quite accurately the nature of 

 some of the remarks which I made on his Essay when I 

 read it (at the request of Dr. Ferrers) in MS. The 

 passage of the last edition of my book, to which he refers 

 as being aimed at him, was meant as a defence or ex- 

 planation of my own procedure. So far as I can recollect, 

 I urged Mr. McAulay to avoid (when possible) dealing 

 with quaternion elements; and to frame relations between 

 surface and volume integrals, &c., from kinematical or 

 other interpretations of their wholes. 



P. G. Tait. 



THE MANUFACTURE OF PAINTER'S 

 COLOURS AND VARNISHES. 



Painter's Colours, Oils, and Varnishes : a Practical 

 Manual. By George H. Hurst, F.C.S. (London : C. 

 Griffin and Co., 1892.) 



THIS work is intended for those who are more imme- 

 diately interested in the manufacture of painter's 

 colours and varnishes ; and the author, as he says in his 

 preface, has in the treatment of his subject endeavoured 

 to combine theory and practice by giving a short account 

 of the theory of the processes which he describes. Con- 

 sidering the range of the work and the great number of 

 subjects dealt with, the information thus afforded is neces- 

 sarily here and there somewhat scanty, and hardly suffi- 

 cient to work upon. At the same time it appears to us 

 that the bulk of the book might with advantage have 

 been reduced by leaving out most of the elementary 

 chemistry, with which nowadays we must suppose any- 

 one would be thoroughly familiar, who intended to embark 

 on colour-making. A colour-maker ought, above all 

 NO. I 261, VOL. 49] 



things, to be a thorough chemist if he wishes to succeed; 

 and those who merely rely on the working of their stock 

 of receipts, secret processes, and rule-of-thumb directions, 

 will have but little chance of success in the future. The 

 author, in his desire to make his work as complete as 

 possible, has also given undue space to the description 

 of numerous colours and processes which are now obso- 

 lete, and to others which attracted only a transient 

 interest when issued from the Patent Office. 



The uninitiated reader will therefore find it somewhat 

 difficult to separate the chaff from the grain, as it is not 

 always clear whether, in the description of the processes 

 and the properties of the colours obtained by them, the 

 author gives his own experience, or merely a transcript 

 of the patent specification or the trade circular. This criti- 

 cism applies more especially to the chapter on white pig- 

 ments. Here it is also to be regretted that the author 

 has not give more prominence to the description of the 

 qualities of the typical whiteleads obtained by the dif- 

 ferent processes, and has not brought them into com- 

 parison with other similar lead compounds, and with the 

 various zinc pigments which have been introduced as 

 substitutes for the poisonous whitelead. The results of 

 such comparative trials carried out systematically, and 

 according to the methods mentioned in the book, would 

 have been of great value, as they would also assign the 

 proper value to those compounds which are so persistently 

 recommended as being equal, or even superior, to genuine 

 old-fashioned whitelead. 



With regard to zincvvhite, it appears to us that the real 

 reason of its not being more generally employed is not 

 its supposed want of covering power, for well-prepared 

 Belgian zincwhite ground in oil is hardly inferior in this 

 respect to whitelead, but that it does not work so smoothly 

 and freely under the brush, however finely it may be 

 ground. On this account the workman will always prefer 

 to use whitelead, and he will only use zincwhite when he 

 is compelled to do so. We notice that the author, in 

 describing the preparation of zincwhite, says " the 

 vapour of zinc burns, and vapour of zincoxide is emitted 

 in large volumes " ; and further on he speaks of the use 

 of " zinc ores, such as calainine and zincblende, and any 

 product of zinc which, being heated in a retort, can be 

 reduced to the metal." Both these statements are 

 absurdly incorrect. Here also we notice that the 

 author, in referring to certain furnace operations, makes 

 frequent use of the term " calcining," but quite indis- 

 criminately, whether the operation is carried out on an 

 open hearth, or in a muffle with the exclusion of air, or 

 in a covered crucible. It is desirable that writers on 

 technical matters should preserve the strict meaning of 

 such terms as this, which have a distinct and specific 

 signification. 



The chapter on Barytes the author introduces with 

 the following words : — " Barytes is one of the most im- 

 portant white pigments at the disposal of the painter, 

 probably in this respect ranking next to whitelead." 

 We have no hesitation in saying that this will surprise 

 those who have any acquaintance with the behaviour of 

 barytes when ground in oii or in varnish. It is true that 

 this substance, as shown in the numerous analyses given 

 in this book, enters largely into the composition of a 

 good many commercial colours ; but we venture to say 



I 



