4.q6 



NA TURE 



[September 20. 1894' 



now out of date, is first to study the transformations of 

 some one substance, such as alcohol, which lends itself 

 to many changes, which are easily traced experimentally, 

 and subsequently to deal with series. This leads 

 naturally and easily to the great object of the detailed 

 study of carbon compounds. .Xpart from their practical 

 utility and the application of knowledge gained by this 

 detailed study to the purposes of the vegetable and 

 animal physiologist, the great aim of the organic chemist 

 is surely to trace the relation of chemical constitution to 

 physical properties, and so to shed light upon the wider 

 question of the constitution of matter generally : but this 

 is just the aspect of the study which, in most of the text- 

 books, is kept in the background. W. A. T. 



PRACTICAL PHYSICS. 



Phvsikalisches Prakticum, jnit besondenr Bcriicksich- 

 tigung dcr physikalisch-chemischen Methoden. \'on 

 Eilhard Wiedemann und Hermann Ebert. Zweite 

 verbesserte und vermehrte .-Vuflage. Pp. xxiv. 455. 

 2/9 Woodcuts. (Braunschweig, \'iewig, 1S93.) 



RECENT years have seen a great development in the 

 teaching of practical physics, and a great increase 

 in the number of laboratories in which instruction in the 

 elementary parts of the subject can be given to large 

 classes of students. So much has this been the case, 

 that now practical physics is taught in a good many of 

 our schools, and forms one of the subjects in numerous 

 examinations. Those who hive been largely concernei 

 in the establishment of classes for practical instruction 

 in physics, and have had some experience in actual 

 teaching, have often felt the need of a suitable book to 

 put into the hands of their students, and have en- 

 deavoured, each for himself, to supply this want. This 

 is the origin of several books on practical physics, such 

 as Glazebrook and Shaw's manual (to take an English 

 example), and the work before us. The authors of such 

 books are able to employ the MS. or the proof-sheets in 

 the instruction of their students, and thus are able to 

 obtain a practical test of their work before sending it 

 forth to the public, with the result that the books are 

 generally very satisfactory for the purposes for which 

 they are designed. The only drawback is that each book 

 is apt to appeal only to a particular type of sludents,and 

 to give descriptions of the apparatus in a particular 

 laboratory. 



The '•Practical Physics" of Wiedemann and Ebert 

 has been designed for a special class of students, viz. 

 those who are chiefly interested in acquiring a knowledge 

 of chemistry. Particular attention has therefore been 

 devoted to those parts of physics which are of most use 

 in a study of chemistry, while several parts of the subject 

 of great interest to physicists have been either omitted or 

 only very briefly dealt with. Thus experiments on rigid 

 dynamici and on the magnetic properties of iron and 

 steel are completely passed over. 



The authors have not attempted to give an account of 

 the methods of precision which may be employed in the ; 

 experiments selected by them, and consequently have i 

 taken no notice of the small corrections which become 

 of so great importance in an accurate research. In 

 NO. 1299, VOL. 50] 



those cases where it seemed desirable that some soui! 

 of error should be brought prominently before ti 

 student's notice, the experiment is so arranged that 1 

 correction shall not be too small. 



To each section there is an introduction wherein t 

 general laws to be employed are stated, and the quantit 

 to be measured are sometimes defined ; but, as a rule, 

 account whatever is given of the reasoning by which t 

 formul.E are arrived at. This is bound to be unsat 

 factory, and must lead the student to be continually a 

 ing for explanations of the formul.c, unless indeed he 

 a person of little mental vigour, when he will acci 

 formul.-e without a murmur. In many instances 

 explanation or definition whatever of the quantity to 

 measured is vouchsafed to the student. For example, 

 is informed that the coefficient of viscosity of a liqi 

 can be determined by the formula 



where \' is the volume of liquid which is driven throu . 

 a fine tube of radius r and length / in time / by a pp 

 sure /, but there is absolutely no definition of 1 

 coefficient of viscosity, although it might have been giv 

 in a few lines. The same complaint may be made ab; 

 several other sections. The authors are surely mistak 

 in their idea that by their method the use of books 

 " the higher mathematics" may be dispensed with, a 

 the " Prakticum " become a self-contained treatise on pr 

 tical physics, wherein the student may find all he requi: 

 without the trouble of searching through special te 

 books. Besides, it is in every way better that the studi 

 should endeavour to acquaint himself with the metho 

 by which the formuLu have been deduced. He gains 

 this way a grasp of the principles of the subject which 

 hardly attainable in any other manner ; and if he d( 

 learn a little m.athematics, he may hope that it will i 

 seriously injure his ability for chemistry. 



Before dealing with the contents of the book, it may 1 

 well to mention some points in which the book is far 1( 

 satisfactory than the authors were capable of making . 

 The first complaint is that the resalts of the sample <t 

 periments are frequently set down without any slatemel 

 of the units in which they are expressed. For instani 

 the modulus of rigidity of brass is found by an expe, 

 ment to be 4770 somethings, but what the somethings i\ 

 is not stated. The student who happened to express 1 1 

 linear magnitudes in centimetres instead of millimetn 

 would doubtless be much perplexed when he fou 1 

 by his experiment the value 477,000 instead of I 

 result in the book. If, on the other hand, the value i 

 the modulus had been expressed as 4770 kilogrammi 

 per square millimetre, all the difficulty would havebe'| 

 avoided. The student should be so trained to state pij 

 cisely the units in which his results are expressed, th 

 the bare statement that the modulus of rigidity is 47, 

 should produce an unsatisfied feeling, in his mind,mui 

 the same as is called up by the conundrum, Why is 

 house.' In some instances where units arc given, th 

 are given wrongly, as when the velocity of sound is fou 

 to be 33 IS metres, and the average velocity of hydrogj 

 molecules is stated to be 1698 metres. j 



A minor defect is that one system of units is n] 



