NA TURE 



{_Nov. 4, 1886 



the contraction whicli ensues on diluting an aqueous solution 

 proceeds continuously, and the molecular volume of a salt in 

 solutions of difterent strengths is continuously greater the larger 

 the amount of salt present. So that in none of these thermal or 

 volumetric phenomena is any discontinuity observed, or any 

 indication of the formation of compounds of definite composi- 

 tion, distinguishable by characteristic properties. 



The question we are now considering, as to whether in a 

 solution the solvent and the substance dissolved in it — or any 

 portion thereof — exist independently of each other, is in some 

 degree answered by the facts known as to the specific heats and 

 vapour-pressures. For instance, when water is added to a solu- 

 tion of sodium nitrate, the molecular heat of the resulting liquid 

 seems to show that all the water added is influenced at least 

 until a very large quantity is present. In this case one molecule 

 of sodium nitrate can afiect the movements of a hundred mole- 

 cules of water, and probably more. It is also well hnown that 

 the vapour-prefsures of water holding in solution almost any 

 dissolved solid is less than the vapour-pressure of pure water, 

 and that the boiling-point of a liquid is raised by the addition to 

 it of any soluble non-volatile substance. This fact of reduction 

 of pressure can only be explained upon the hypothesis that there 

 is no free water present at all ; that is, that there is no water 

 present which is not more or less under the influence of the 

 dissolved substance. 



What becomes of water of crystallisation forms a part of the 

 same question as to the relation of solvent to solvent. Observed 

 facts lead us to conclude that white copper sulphate, blue 

 anhydrous cobalt chloride — and, by analogy, other salts which 

 are colourle-s — retain their hold upon water of crystallisation 

 when they are dissolved in water. A very important observa- 

 tion has been made by Dr. Nicol which bears directly upon this 

 question. In his study of the molecular volumes of salt solutions 

 he finds that, when a salt containing water of crystallisation is 

 dissolved, this water is indistinguishable by its volume from the 

 rest of the water of the solution. In the report presented to the 

 British Association last year, the following passage occurs : 

 " These results point to the presence in solution of what may be 

 termed the anhydrous s.alt in contradistinction to the view that 

 a hydrate, definite or indefinite results from solution ; or in other 

 words, no part of the water in a solution is in a position rela- 

 tively to the salt difterent from the remainder."' 



These two statements, however, are not strictly consequent 

 upon each other. The view seems preferable that (save, per- 

 haps, in excessively dilute solutions) the dissolved substance is 

 attached in some mysterious way — it matters not whether it be 

 supposed to be chemical or physical — to the whole of the water. 

 We cannot otherwise get over the difficulty presented by the 

 hydrated salts, which give coloured solutions, by the control of 

 the vapour- pressure of the dissolved salt, and by the altered 

 specific heat. With regard to water of crystallisation, E. 

 Wiedemann has shown that hydrated salts in general expand 

 enormously at the melting-point ; and the observations of 

 Thorpe and Watts on the specific volume of water of crystaUis- 

 ation in the sulphates of the so-called magnesium group show 

 that, whilst the constitutional water occupies less space than the 

 remaining molecules, each successive additional molecule occu- 

 pies a gradu.ally increasing volume. So that when a salt, .with 

 its water of crystallisation, passes into the liqmid state (either 

 by melting or by solution in water), it requires a very slight re- 

 laxation of the bonds which hold the water to the salt for it to 

 acquire the full volume of liquid water, whilst the water of con- 

 stitution is not so easily released. And this conclusion accords 

 wih Nicol's observations on the molecular volumes of the salts 

 when in solution. 



Now comes the question as to what determines the solubility 

 of a substance. Why, for example, is magnesium sulphate very 

 soluble in water, whilst barium sulphate is almost totally in- 

 soluble? With regard to salts the following propositions seem 

 to be true : — (i) Nearly all salts which contain water of crystal- 

 lisation are soluble in water, and for the most part are easily 

 soluble; (2) insoluble salts are almost always destitute of water 

 of crystallisation and rarely contain the elements of water ; (3) 

 in a series of salts containing nearly allied metals the solubility, 

 and capacity for uniting with water of crystallisation generally, 

 diminish as the atomic weight increases. 



The fusibility of a substance has also much to do with its 

 solubility. Neither fusibility alone nor chemical constitution 

 alone seems to be sufficient to determine whether a solid shall 

 be soluble or not. But it may be taken as a rule to which there 



are no exceptions that when there is a close connection in 

 chemical constitution between a liquid and a solid, and the 

 solid is at the same time easily fusible, it will also be easily 

 soluble in that liquid. 



Salts containing water of crystallisation may be considered as 

 closely resembling water itself, and these are for the most part 

 both easily fusible and easily soluble in water. But space is 

 wanting for the discussion of the details of these matters, as well 

 as of the relation of molecular volume to fusibility of solids. 



The fascinating character of the phenomena of supersatura- 

 lion has attracted a host of experimenters, but no definite 

 explanation has been generally accepted. In the opinion of the 

 speaker supersaluration is identical with superfusion. Super- 

 saturated solution of, say, alum, thiosulphate of sodium melted 

 in its water of crystallisation, and fused sulphur at ICX3°, exhibit 

 phenomena of exactly the same kind. 



Finally, we are led to the consideration of what is n:eant by 

 chemical combination. From the phenomena under discussion, 

 and others, the conclusion seems inevitable that chemical com- 

 bination is not to be distinguished by any absolute criterion 

 from mere physical or mechanical aggregation ; and it will 

 probably turn out ultimately that chemical combination differs 

 from mechanical combination, called cohesion or adhesion, 

 chiefly in the fact that the atoms or molecules of the bodies 

 concerned come relatively closer together, and the consequent 

 loss of energy is greater. 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE 



Cambridge. — Of the students in Natural Science entered at 

 Cambridge this term no fewer than 116 have already announced 

 their intention of studying medicine. 



Dublin. — The Senate of the Royal University has conferred 

 the degree of Doctor of Science honoris causa upon James Bell, 

 Ph.D., F. R.S., Principal of the Somerset House Laboratory. 



SCIENTIFIC SERIALS 



Revue cV Anthyopologie, troisieme serie, tome I, Paris, 1S86. — 

 On the Simian characters of the Naulette jaw, by M. Topinard. 

 This celebrated find, which was discovered at the bottom of an 

 obscure cavern 25 m. below the present level of the Lesse, near 

 Dinant, in Belgium, is chiefly remarkable for its excessive 

 prognathism, which is due alike to the great thickness of the 

 horizontal branch of the jaw when compared with its height, 

 and to the special obliquity of the axis of the alveolus of the 

 second molar. In its relative proportions the Naulette jaw 

 must be characterised not only as non-human, but as plus- 

 Simian. A careful comparison of the Naulette jaw with the 

 maxillai7 processes of the anthropoids, and of several of the 

 lowest extant human races, has led M. Topinard to the conclu- 

 sion that in the age of the mammoth, tichorine rhinoceros, and 

 cave-bear, there had already appeared numerous mixed human 

 types, to one of the lowest of which it may be presumed that 

 the Naulette jaw belonged. — On the population of Banibouk, 

 on the Niger, by Dr. Colin. An interesting paper on an exten- 

 sive, but very imperfectly-known, region of Western Soudan, 

 exclusively inhabited by a branch of the great Manding race, 

 known as the Mali-nkes. The Bambouk territories, more than 

 600 kilometres in length, and from 80 to 150 in width, are 

 divided into numerous little States, most of which enjoy a com- 

 plete autonomy. Their want of consolidation, and the indiffer- 

 ence of the people to all forms of religion, have made the Mali- 

 nkes objects of contempt to their Mussulman black neighbours, 

 but according to the narrations of the Griotes, or itinerant 

 bards, who are to be met with in every part of Western 

 Africa, they had at one time extended their dominion 

 over all the trilies on the right banks of the Niger, and 

 were preparing to invade Saigon when the advance of the 

 French forced them to fall back within their original limits. For 

 a time they submitted to the restrictions of Mohammedanism, but 

 now they appear to have absolutely no religion. They prepare 

 an intoxicating drink from honey, called "dolo," in which 

 women as well as men indulge to excess. The men are indo- 

 lent, hunting only to avert starvation, and working their exten- 



