June 6, 1895] 



NATURE 



12: 



Part ii. is de\ oted cxclusi\ cly to mules and mule breed- 

 ing, and is replete with valuable and exhaustive informa- 

 tion on these subjects. The authors strenuously deny the 

 existence of fertility in either the male or female mule, 

 affirming that abnormal lactation not unfrequently occurs 

 in female mules, when milk is secreted in great abund- 

 ance, and that the foals which they arc observed to be 

 suckling are in reality the foals of other animals which 

 the mules have adopted. With regard to the oft-quoted 

 instance of a mule in the Acclimitisation dardcns in Paris, 

 which has produced foals when mated both with the 

 horse and ass, the writers doubt whether the animal is a 

 mule, and assume that she is an ordinarv' mare, whose 

 female parent was influenced by a first alliance, as is so 

 often the case in dogs and other animals. If their con- 

 tention is correct, the mule may still aptly be described as 

 "an animal of no ancestry and with no hope of posterity." 



The writers are enthusiastic, nay e\en fulsome in their 

 praise of this hybrid, and bitterly lament the lack of 

 appreciation in which it is held in (Jreat Britain as com- 

 pared with America and some European .States. " In 

 endurance,' say the authors, '"capability of hard labour, 

 economy in keep, longevity, and freedom from disease, 

 mules far surpass horses." Into so controversial a matter 

 this is not the place to enter, and we must content our- 

 selves with the belief that so plain and oftentimes so ugly 

 an animal as the mule will never supplant to any great 

 extent, in this country at least, the beautiful and graceful 

 varieties of the horse of which Englishmen are naturally 

 so proud. 



To any of our readers who are interested in the subject 

 of mule breeding, this work may be heartily recommended : 

 and, in conclusion, we feel bound to compliment in the 

 highest terms all who have been instrumental in its 

 production. W. F. ( 1. 



The Moon. By T. (iuyn Elger, F.K..-\.,S. Pp. 174. 



(London : (icorge Philip and Son, 1895. 

 I.\ this latest work on the moon, from the pen of one 

 of the foremost of British selenographers, the most note- 

 worthy feature is the excellent chart, eighteen inches in 

 diameter ; this is given in four quadrants, but it can also 

 be obtained complete and separately. .^11 the named 

 formations are distinctly shown, and the names of the 

 more important are very clearly printed on the map itself 

 The greater |)art of the text resohes itself into a descrip- 

 tixe index to the map ; but though this appears in rather 

 stereotyped fashion, it embodies a good deal of inform- 

 ation which has been gleaned by the author during many 

 years of observation. An introduction of forty pages 

 deals with lunar phenomena generally, and includes 

 numerous hints which will be of use to the observer. Mr. 

 KIger objects most emphaticalh to our satellite being 

 spoken of as a changeless world, and justifies his jjosition 

 b\ stating that volcanic outbursts, producing mountains 

 as large as the .Monte Nuovo, might occur in many parts 

 of the rnoon without the world being any the wiser. Though 

 possessing little of novelty, and not appealing to the 

 general reader, the book and map together constitute a 

 handy work of reference which obseners of experience, 

 as well as beginners, will be glad to have by them. A ' 

 few details as to the phenomena to be observed during 

 eclipses of the moon, might have been included with 

 advantage. 



Algebra. Parti. By .M. H. .Senior, i ( Hdham : I). \V. 



Hardsley.) 

 KlNi)F,K(;.\KTi;x methods of teaching are now applied to 

 most subjects. In this small book of fifty pages, the 

 author endeavours to make algebra interesting to young 

 students by associating the abstract symbols with con- ' 

 Crete objects. The no\el features of the book are the 

 explanation of brackets, the exercises on factors, short 

 methods of multiplication and di\ision, the elucidation of 

 signs, and the numerous practical examples. I 



NO. 1336, VOL. 52] 



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 of NATURE. 

 No notice is taken of anonymous communications.'\ 



Argon and Dissociation. 



Thk discovery of the new substance argon, by Lord Rayleigh,' 

 has given rise to a difficulty which, it is thought by some, shows 

 that the periodic law of Mendelejelf has not that generality 

 which has been attached to it by chemists during the last few 

 years. 



According to Lord Kayleigh"s determination, the density of 

 argon is ig'g (H = l), making the atomic weight 39-8, as the 

 molecules are shown to have no internal energy of the same 

 order as their energy of tran.slation, and hence to bemonatomic. 

 Argon with this atomic weight cannot possibly find a place in 

 the natural classification. If its atomic weight were less than 

 39'i (the atomic weight of pota.ssium), argon would fall in the 

 Vlllth or interperiodic group in Lothar Meyer's table ; and its 

 properties, so far as they have been investigated, would harmonist 

 with this position. 



The determination of the vapour, density of iodine by A'. 

 Meyer, Crafts and Meier, and others, has showTi that at tem- 

 peratures below 1000 C. the gas consists of diatomic molecules, 

 while above this dissociation takes place, and above 1500° C. we 

 have the dissociation complete, and the molecules are mon- 

 atoniic. 



Why, then, cannot we have a similar behaviour in the case of 

 argon ? 



If argon at low temperatures (somewhere near its critical 

 point) consisted of diatomic molecules, which dissociate as the 

 temperature rises, the difficulty of the position of argon would be 

 removed. Thus, suppose at the temperature at which I9'9 was 

 determined as the density of argon, the dissociation has pro- 

 ceeded so far that 5 per cent, of the molecules remain diatomic ; 

 the average molecular weight w ould be 39'8, but we should have 

 two kinds of molecules, monatomic and diatomic, and the atomic 

 weight under these supposed conditions would be 



39-8 X 100 _ „ 



The ratio of the specific heats, at constant pressure and con- 

 stant volume, taking i -4 for this ratio for a gas w ith diatomic 

 molecules, and j for a gas with monatomic molecules, would be 

 fiir argon, on the above supposition, 



9S_xJ_ + _5xi "4 _ 



I -6?. 



This value agrees very well with the values (1-16 — 1-65) 

 determined for argon. 



This explanation reconciles argon with the natural classifi- 

 cation ; and as yet no facts have been published in opposition 

 to it. 



If this hypothesis be true it could be easily verified, for at 

 temperatures, not much higher than that at which the vapour 

 density determinations were made, the dissociation would be 

 complete : and hence the vapour density in agreement with a 

 molecular weight about 38: and also at lower temperatures 

 than that at which the vapour den.sity has been determined the 

 gas wnuld not obey Charles' law ; for the recombination of the 

 single atoms to form diatomic molecules, and possibly molecules 

 containing a greater number of atoms, would cause a contraction 

 greater than that due merely to the cooling of the gxs according 

 to the ordinary law. Phnrv \'.\ii;han Bevan. 



.Melbourne University, .\pril iS. 



Prof. Be\an ascribes to me work done conjointly with Prof. 

 Kamsay. .An addendum to our jrapcr (see Proc. Koy. Soc.) con- 

 tains our account of ex|K.Timents by Prof, kamsay, especially 

 directed to examine the question raised. 



It has turned out that the gas possesses the .same value of 



-^ as hydrogen, and that the value of this expression .is not 



altered between - 90" and -f- 250°. The most trustworthy deter- 

 mination of the ratio of specific heats gives the number i '65 ;• 

 but u\uch dependence is not to he placed on the accurate value 



