February 15, 1894] 



NA TURE 



363 



into two main divisions. In the present work, although 

 the notation of the integral calculus has not been 

 used, yet the method of integration has been explained, 

 and, in fact, applied to the solutions of some problems, 

 ■such as those of finding moments of inertia, centres of 

 pressure, &c. The treatment of the subject-matter on 

 the whole has, however, been developed by very simple 

 mathematical methods, and although the book is 

 published in the " Advanced Series," it should not for that 

 reason be reckoned as of too high a standard for 

 elementary readers. Indeed the author has inserted 

 several useful introductory chapters to prepare such 

 readers as those who have only an elementary knowledge 

 of the mechanics of solids ; thus we are treated to 

 chapters on units, principles of statics, uniform, circular, 

 and harmonic motions. In these chapters, and also in 

 the others dealing with the mechanics of fluids, the 

 author seems to be especially clear in his explanations, 

 and his remarks are in many cases accompanied with 

 diagrams and illustrations, which are always of great 

 help to the reader studying the subject for the first time. 



In a work of this kind a student can best obtain a good 

 grasp of the subject by supplementing his study of the 

 text with the working out of numerous typical examples. 

 Those here inserted should be specially useful in this 

 direction, and in many cases they have been divided into 

 two series, the second being of a more difficult type than 

 the first ; many of the examples are taken from such 

 sources as the examination papers set at South Kensing- 

 ton, and those for the Civil Service and the Universities. 



As a text-book for science schools, and suitable for 

 those wishing to get a thorough insight into the subject, 

 the book will be sure to find favour. 



Hozv to Ma7iage the Dynamo. By S. R. Bottone. 

 (London : Whittaker andXo.) 



In this book the author gives, in thirty-five short pages, 

 a few practical directionsfor the installation and manage- 

 ment of a dynamo. The instructions given are intended 

 for the use of engineers who have no knowledge of elec- 

 trical work, but are called upon to undertake the manage- 

 ment of a dynamo. It is hardly possible in such circum- 

 stances to frame directions which can be intelligently 

 carried out. The dynamo attendant, like every other 

 person in charge of machinery, must really learn by an 

 experience which no manual can replace. The instruc- 

 tions in the text are, however, plainly and simply stated, 

 and deal with some of the more important points con- 

 nected with the care of dynamos. 



An appendix is devoted to the explanation of technical 

 terms used in electricity, and makes up about twelve of 

 the total forty-seven pages of the book. It can hardly 

 be regarded as satisfactory, either in point of accuracy or 

 extent. It is more of a series of detached explanatory 

 statements than a " table of definitions," as the author 

 calls it in the preface. 



LETTERS TO THE EDITOR. 



The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part ^/Nature. 

 No notice is taken of anotiytnous communications.'] 



The Cloudy Condensation of Steam. 



(i) The fault I have to find with Mr. Bidwell (Nature, 

 December 28, p. 212, C893) ^^^ others who are aiming to 

 revive the chemical hypothesis tentatively broached by the late 

 R. von Helmholiz (Wied. Ann. xxxii. p. i, 1887. r/. p. 7 et 

 seq.), is that these gentlemen are abandoning a tried theory 

 when its capabilities are far from exhausted. To begin at once 

 with the most recent contribution to our knowledge of the 

 subject, Mr. Bidwell's observation, let a series of excessively 



NO. 1268, VOL. 49] 



small condensation nuclei be enclosed in a vessel without further 

 interference. They were active before being put there ; but 

 after remaining imprisoned for a sufficient length of time, 

 Mr. Bidwell finds them shorn of their power to produce con- 

 densation in supersaturated steam. Now, I ask, is it at all 

 probable that these particles will remain distinct throughout the 

 whole time of confinement? I think not. Take the familiar 

 case of fine clay suspended in water. What goes on during 

 subsidence is an agglomeration of particles. This process may 

 be enormously accelerated by small additions of acid or almost 

 any other foreign matter to the water. In ether the speed of 

 agglomeration is so marked, that the (dry) mud which may be 

 suspended in water for weeks, or even months, falls out of ether 

 like so many grains of sand. Witness, furthermore, the case of 

 any chemical precipitate. Those little solid corpuscles were 

 originally all but molecular in size ; yet they grew with such 

 enormous rapidity to so enormous a bulk (relatively speaking), 

 that the formed precipitate subsides quickly (in most cases) 

 even in water. Think of those monstrous clots of fresh chloride of 

 I silver ; think, too, that they were being built up of individual 

 [ molecules in an instant and before one's very eyes. Then 

 I why should condensation nuclei be so good as to remain distinct 



indefinitely? 



i In brief, it is not probable that the condensation nuclei^ if 



I once produced can subsist in their original degree of comminu- 



, tion ; for whatever the nature of the acting forces may be, this 



I finely divided matter is in a highly potentialised state relatively 



to coarser matter, and there will be dissipation of energy by 



mere mechanical agglomeration even when chemical action is 



excluded. 



(2) Let us look for further confirmation at the size which these 

 I active nuclei must necessarily have, remembering that the con- 

 densation in question, when properly vievved, always appears 

 coloured.'- Such condensation presupposes droplets nearly all 

 of the same size ; of a size, moreover, which we can reasonably 

 conjecture to lie somewhere between '000,004 centimetre and 

 ■000,040 centimetre, depending on the colour selected. This 

 implies that the nuclei which induce coloured condensation must 

 all be of the same size (certainly an improbable condition), or that 

 they must be small even when compared with the small droplets 

 stated. If it were not so, large drops would condense on large 



\ nuclei, and small drops on small nuclei (keeping in mind that 

 the whole process of condensation is virtually instantaneous), 

 and there could be no prevailing dimension, and consequently no 

 intense colour. 



But it will be asked, will these excessively small particles 

 meet the Kelvin conditions of condensation ? Assuredly. One 

 may estimate that pure, dust-free, unconfined steam at 100° would 



■ require a pressure of the order of ten or more atmospheres to 

 condense it. Add to this dust particles less than 'ooOtOOl cen- 

 timetre in diameter, and the pressure sinks to 15 centimetres of 

 mercury ; in case of particles "000,010 centimetre in diameter, to 



i one or two centimetres of mercury ; in other words, to pressure 

 increments •* certainly met with in steam jets. The fact that 

 nuclei of a few hundred molecular diameters are needed is the 

 very feature of these experiments, and explains why smoke and 

 other coarse material is useless, and why the condensation- 

 producing dust must be so highly specialised. 



(3) Nuclei, it is true, are often equally active if derived from 

 liquids, the mixture obtained by passing air over very strong 

 sulphuric acid being a notable example. Let us remember, how- 

 ever, that these active liquid bodies (acids all of them, while 

 ammonia is not active) may form sulphate of ammonia in contact 

 with atmospheric air. But if this view of the case be unsatis- 

 factory, is there anything in Lord Kelvin's well-known equation 

 connecting vapour tension and curvature which asserts that the 

 nuclei 7nust be solid ? 



(4) I have equally great difficulties in submitting to the diverse 

 electrical hypotheses put forth, and I cannot understand why so 

 astute and sound-minded an investigator as Mr. Aitken should 

 hazard his birthright for that mess of electrical pottage. It is 

 certain that air passing across an active spark gap will produce 

 condensation ; but it is equally certain that no electric field will 



1 It must not be forgotten that the imprisoned nuclei are mixed with the 

 nuclei normally present in air. Whether the mixture is pronounced active 

 o\ not, depends on the sensitiveness of the test applied. Thus there is room 

 for an error of judgment. I recur to this in § 6. 



2 I here include the op.ique field mentioned in Mr. Aitken's and in my own 

 articles, since it obeys clean cut colour laws. 



3 Pressures between i and So centimetres above the atmosphere were 

 necessary in my work. 



