172 



KNOWLEDGE 



[September 1, 1891. 



Manual of Assayin;/. By Walter Lee Brown, B.Sc. ; 

 Revised, CoiTected, and considerably Enlarged by A. B. 

 Griffiths, Pli.D., F.R.S. (London: W. Heinemann. 

 1H90. ) Mr. Brown's manual, of which this work is a 

 revised edition, is well known to practical assayers, though 

 its reputation has probably been greater in the United 

 States than in this country. Dr. Griffiths has improved 

 the work in several respects, and has added an important 

 chapter on Fuel. We could have wished that more use 

 had been made of the great work by Dr. Percy, certainly 

 a higher authority than Mitchell's manual, to which the 

 author frequently refers ; and perhaps some of the illus- 

 trations might with advantage have been replaced by 

 new ones. One drawback with regard to the drawings repre- 

 senting instruments and apparatus is that they are not on 

 anything like a uniform scale— no one could form a correct 

 idea of the relative sizes of the things themselves from the 

 woodcuts : I'.i/., the hammer and anvil on page 69, and the 

 cupel on ijage 63. The different processes employed by 

 the assay er are carefully and clearly described, and the 

 various possible causes of failure in obtaining trustworthy 

 results are, in most cases, sufficiently indicated. Part I. 

 deals with apparatus and re-agents, and is divided into 

 three chapters, in which descriptions are given of the 

 implements used for pulverizing, sampling, &c., scales and 

 balances, weights, furnace tools ; also wet and dry re- 

 agents for assaying. Then follow descriptions of certain 

 processes, such as the testing of lithage for silver, the 

 testing of lead for silver, and the determination of the 

 reducing and oxidizing of certain agents. These descrip- 

 tions, which are thoroughly practical, are largely helped 

 by numerous Olustratious of apparatus and instruments. 

 Perhaps the most valuable feature of the book is the way 

 in which the numerical examples of assaying are worked 

 out in several places in the body of the treatise itself and 

 in the appendix. These will doubtless be appreciated by 

 such students as may be endeavoimng to prepare them- 

 selves by self-tuition for future practical work as assayers. 

 We quote the following passage as an example to show the 

 practical nature of the book : " Absolutely accurate assays 

 of gold and silver bullion require care, skill, and first-class 

 apparatus. The skill may soon be acquired by practice, 

 but the apparatus must not only be of the best quality, but 

 must be kept in the most perfect state of adjustment. It is 

 not enough to purchase chemicals which are marked ' pure,' 

 or a balance supposed to be accurate : the chemicals must 

 be tested, and the accuracy and adjustment of the balance 

 and weights verified, before correct results can be obtained." 

 The appendix concludes with lists of the principal gold, 

 silver, copper, and lead minerals, a list of useful books 

 connected with the subject, a plan of a laboratory, outfit, 

 tables of weights and assay ton equivalents. 



Coal, anil what we c/i-t from it : a Romance of Science. 

 By E.tPHAEL Mendola, F.R.S. This very instructive little 

 treatise is one of the " Romance of Science Series," 

 published by the S.P.C.K., and the name of its author is 

 sufficient guarantee for thoroughness and accuracy. 

 Though some readers might perhaps be somewhat repelled 

 at first by the long names given to carbon compounds, yet 

 we think this prejudice will be overcome as they peruse its 

 pages, and give place to a keen sense of delight as they 

 follow the marvellous transformations so ably described by 

 the author. The earher chapters of the book contain a 

 clear and interesting account of the origin of Coal, and of 

 the energy stored up m it, illustrative of George Stephenson's 

 celebrated statement about " bottled-up sunshine." This 

 is followed by a sketch of the history of coal gas, together 

 with an account of its manufacture as at present carried on. 



The author next takes up the subject of the commercial 

 importance of the liquid and solid products resulting from 

 the distillation of coal, viz. — the " ammoniacal liquor," the 

 coke, and the tar. The second half of the book is, in fact, 

 occupied with the history and description of the marvellous 

 range of substances obtained, directly or indirectly, from 

 coal tar. In the early days of gas manufacture, this black 

 unsavoury residue was, in every sense, a " waste product," 

 and the manufacturer was only too glad to get rid of it as 

 best he could. Now it is made to furnish not only an 

 almost endless variety of brilhant colours, but delicious 

 essences and delicate perfumes, while immense fortunes 

 have been realized by the fortunate discoveries of the 

 processes by which such results have been obtained. 



In 1828, Faraday discovered his " Bicarburet of 

 Hydrogen " in the oil produced by the condensation of the 

 so-called " oil-gas." In 18:-5i, the same hydro-carbon was 

 obtained by Mitscherlick by heating benzoic acid with 

 lime, and since then it has been known in this country as 

 benzol or benzene. In 184.5, Hofman proved the existence 

 of benzine in the light oils from coal tar, and in 1858, 

 Perkin took out his first patent for a coal-tar colour under 

 the name of " mauve." For the wonderful development of 

 this branch of chemistry, and for an account of its extension 

 to medicine, sanitary science, photography, and the ai'ts, 

 we must refer the reader to the book itself, which will well 

 repay careful reading, and put him in possession of the 

 latest processes and discoveries. 



Prof. Max Wolf, of Heidelburg, has obtained some 

 interesting photographs of the Milky Way, in the neigh- 

 bourhood of a, Cygnus, which we hope to reproduce in 

 the October number. 



Utttrrs. 



[The Editor does not hold himself responsible for the opinions or 

 statements of correspondents.] 



OX THE COIIPARISOX OF PHOTOGRAPHS OK THE 

 MILKY WAY. 



To the Editor of Knowledge. 



Dear Sir, — In the letter on the above subject which 

 appears in the May number of Knowledge, Mr. Barnard 

 raises a question of great importance. The words used 

 are, " I have no hesitation in attributing the difference 

 between these pictures entirely to the development of the 

 negative. As I have taken occassion to remark elsewhere, 

 the utmost care must be exercised in the development of 

 MUky Way pictures to bring out the cloud -forms clearly 

 and strongly." This is a point that forced itself upon my 

 notice as soon as I began taking photographs of the Milky 

 Way, for the details in these pictures are often as delicate 

 as a thin fog on the plate, and the sUghtest over-developing 

 may suffice to produce such marks, which it is almost 

 impossible to distinguish from features of the Milky Way. 

 Indeed, I only know of one method of making sure which 

 parts of the negative show faint ]\Iilky Way features and 

 which are fog, and that is to multiply the pictures and 

 compare them carefully. Now, my photograph of the Jlilky 

 Way about « 17h. 58m. and 8 30', which you reproduced 

 in Knowledge for March last, is from one of eight negatives 

 of this object which I have taken, and it is not the best so 

 far as the nebulous part is concerned, but they all agree in 



