NA TURE 



[February 27, 1908 



ratio of the temperature of inversion to the critical 

 temperature ; in a number of cases the ratio has the 

 value 298, but this figure, though substantially con- 

 stant, differs considerably from the theoretical value 

 27/8 = 3-37 (p. 68). In a third table (p. 83) are given 

 the minimum volumes for a series of gases under ex- 

 treme pressure and at low temperatures, as compared 

 with the volumes at the critical point of each gas; 

 the actual values : 



O2 C'l., CO2 SO2 CjHj CClj CjHj.iO CiiTr,; 



0-278 0'2S2 0269 0243 0243 0'260 0'255 0'25S 



a-e nearly constant at o'26, but differ widely from the 

 theoretical value i/3 = o'33. A fourth table (p. 71), 

 in reference to the minima in the \>vlv curves, shows 

 a precisely similar result — the four constants which 

 are given for each of three gases agree closely to- 

 gether, but differ widely from those calculated from 

 the equation of state. 



In view of the failure of van der Wanls to give an 

 exact quantitative explanation of the behaviour of 

 liquids and gases, it is natural that many attempts 

 should have been made to correct and improve the 

 original equation. These attempts are described in 

 chapters xii. and xiii., but the fact that the author 

 has found it necessary to discuss something like a 

 dozen different equations is in itself sufficient evidence 

 that the goal has not yet been reached. 



A part of the difficulty which arises in applying 

 equations such as that of van der Waals is due to the 

 fact that in compounds such as water and the alcohols 

 liquefaction is accompanied by the formation of 

 molecular aggregates (chapter xi.); attention has 

 therefore been directed in recent years mainly to the 

 study of hydrocarbons and similar substances in which 

 this tendency is at a minimum. In most cases the 

 polymerisation is instantaneous, but the author is in- 

 correct in supposing (p. 52) that this is alwa'ys the 

 case ; thus the recent observations of Bamberger and 

 Seligman have shown that in the case of nitroso- 

 butane the association and dissociation (unlike those of 

 nitrogen peroxide) proceed quite gradually. There 

 can be little doubt, however, that he is right in attri- 

 buting the anomalous densities observed by de Heen 

 and others to the presence of impurities rather than 

 to slow changes of molecular aggregation as postu- 

 lated by Traube ; on this point the evidence afforded 

 by the author's own experiments, supplemented by the 

 recent discussion of Verschaffelt, appears to be con- 

 clusive. T. M. L. 



AMERICAN FORAGE CROPS. 

 Forage Crops for Soiling, Silage, Hay and Pasture. 

 By Dr. Edward B. Voorhees. Pp. xiii + 384. (New 

 York : The Macmillan Company ; London : Mac- 

 millan and Co., Ltd., 1907.) Price 65. 6d. net. 



THIS book is one of the Rural Science Series, edited 

 by L. H. Bailey, and designed to give the 

 American farmer simple but accurate instruction in 

 scientific agriculture. Some of the series, e.^. King's 

 " Soil," are of general interest, and are well known 

 here ; the others refer mainly to American conditions, 

 and appeal less to English readers. 

 NO. 2000, VOL. 'JJ~\ 



Forage crops are those which are fed in the green 

 state to animals instead of being left to ripen and 

 produce seed : turnips, mangolds, and " temporary " 

 grass are all examples. They play a highly impor- 

 tant part in every scheme of general farming; indeed, 

 their introduction into England in the seventeenth and 

 eighteenth centuries not only revolutionised our agri- 

 cultural practice, but had a considerable indirect effect 

 on the social life of our ancestors. 



The author deals with practically all the fodder 

 crops grown in the United States, giving details of 

 cultivation and manuring, and, in some cases, sum- 

 maries of the results obtained with the crop at the 

 various experiment stations. These summaries are 

 perhaps the best part of the book, and will be appre- 

 ciated both by the student and the farmer ; we should 

 like the author to have extended them by including 

 short descriptions of the soil and climatic conditions. 

 The cultivation and manurial details are treated 

 mainly from the empirical standpoint, and here we 

 cannot help feeling that the author has missed an 

 opportunity. Details are of very great importance in 

 agriculture, but they should hardly be given the chief 

 place in a text-book like the present one. Neither the 

 student nor the practical man can make much of 

 them; there is an endless variety about them, and a 

 scheme that works well on one farm may not prove 

 suitable on another close by. What is wanted is a 

 clear statement of the general requirements of the 

 crop, followed by a few well-chosen detailed illustra- 

 tions. In this way the student gets a real picture 

 that will be of service to him, and the practical man 

 is put in a position to see whether or not he can profit- 

 ably grow the crop. Unfortunately, the author does 

 not quite give us this, and if the schemes he suggests 

 fail, the farmer is not in possession of the principles 

 which would aid him to frame a modification suited 

 to his land. 



A perusal of the book brings out very clearly the 

 differences between English and American practice in 

 regard to fodder crops. The difference depends not 

 only on climate, but also on labour supply, for in the 

 latter respect the American farmer is worse olf even 

 than his English cousin. We learn, for instance, that 

 one of our best root crops, the mangold, is not widely 

 grown because of the labour required. Its place is 

 taken by green maize, which is partly fed green and 

 partly made into silage. 



The author also deals with the different ways of 

 using fodder crops — soiling, ensilage, and conversion 

 into hay. Soiling was introduced into the States from 

 Europe in the middle of the last century by 

 Josiah Quincy, whose writings on the subject 

 are so good that it is a pity the author 

 makes no mention of him. The practice con- 

 sists in cutting and carrying green crops to the 

 animals, which are kept in stalls all the year round 

 instead of going out to grass in summer. It has 

 proved to be economical in places where land is dear 

 and labour cheap, but is not likely to displace ensilage 

 in America, notwithstanding the prominent place 

 assigned to it in the book. Indeed, one hardly sees 

 how silage could be improved upon for the American 

 farmer ; he has learnt how to make it, and as a 



