64 



NA rURE 



S^Nov. 1 8, 1886 



either bow, e.g. from the cathead, and allowed to tow in tlie 

 water. 



" With the wind on the quarter, the effect seems to be less 

 than in any other position, as the oil goes astern while the waves 

 come up on the quarter. 



' ' Lying-to, the weather bow and another position farther aft 

 seem the best places from which to hang the bags, with a 

 sufficient length of line to permit them to draw to windward 

 while the ship drifts. 



"Crossing a bar with a flood tide, oil poured overboard and 

 allowed to float in ahead of the boat, which would follow with 

 a bag towing astern, would appear to be the best plan. As 

 before remarked, under these circumstances the effect cannot be 

 so much trusted. 



" On a bjr with the ebb tide, it would seem to be useless to 

 try oil for the purpose of entering. 



"For boarding a wreck, it is recommended to pour oil over- 

 board to windward of her before going alongside. The effect in 

 this case must greatly depend upon the set of the current and 

 the circumstances of the depth of water. 



" For a boat riding in bad weather from a sea-anchor, it is 

 recommended to fasten the Ijag to an endless line rove through 

 a block on the sea-anchor, by which means the oil is diftused 

 well ahead of the boat, and the bag can be readily hauled on 

 board for refilling if necessary. " 



ON THE INTENSITY OF REFLECTION FROM 



GLASS AND OTHER SURFACES^ 

 'X'HE author pointed out that mo^t previous experimenters, 

 ■*• especially Rood, had measured the amount of the 

 transmitted light, and that any percentage of error in this 

 measurement was greatly multiplied when the results were used 

 to calculate the amount of reflected light. In his experiments 

 the amount of reflected light was measured directly. The 

 method was as follows. Light from a cloud was passed through 

 ground glass in the window of a darkened room, and made to 

 fall at the polari-ing angle on a plate of glass. The transmitted 

 and reflected rays were conducted along different paths by a 

 series of reflectors, but finally emerged side by side and of equal 

 intensity. One of the reflectors in the path of the reflected ray 

 was the glass surface to be tested, the light falling on it at 

 an almost perpendicular incidence. This glass was now re- 

 moved, and a single mirror was shifted so as to make the angles 

 and points of incidence of the reflected ray on the several 

 mirrors the same as before. The reflected ray was now brighter 

 than the transmitted. To re-establish equality a disk with holes 

 in a ring round the centre was rotated in the path. The ratio of 

 the sum of the breadths of the holes to the whole circumference 

 of the ring gave the percentage of the light that was reflected. For 

 apiece of optically-worked blackened glass the amount reflected 

 was '058 of the total incident light. It was found that the amount 

 of reflection depended greatly on the clearness and polish of the 

 surface. Thus in one case re-polishing increased the amount 

 from '04095 to '0445. Fresnel's formula gave in this case 

 •04514. Generally it appeared that the amount reflected was 

 less than according to Fresnel's formula — a result contrary to 

 that of Rood's. The numbers for polished glass and for silver 

 on glass were '94 and "83. 



ON THE NATURE OF SOLUTION - 

 1 N connection with the discussion on the " Nature of Solu- 

 tion," in Section B, at the Birmingham meeting of the British 

 Association, the following paper was read by Spencer Umfreville 

 Pickering, Professor of Chemistry at Bedford College : — 



The "hydrate" theory attributes dissolution to the existence, 

 in a stable or partially dissociated condition, of definite liquid 

 compounds (generally unknown in the solid form) of the sub- 

 stance dissolved and its solvent, and the mixing of these com- 

 pounds with excess of the solvent. 



In certain special instances we have direct evidence of the 

 reality of such compounds,^ but it is on general grounds rather 

 than on any special experiments that I would seek to establish 

 their existence. 



' Abstract of a Paper read at the Birmingham meeting, i386, of the British 

 Association, by Lord Rayleigh. 



2 Continued from p. 22. From the Chemical Mtnvs- 



3 See especially Berthetot, Ann. Chim. Phys.{^\ 4, 445 to 537. 



There is, in the first place, a strong prima facie improbability 

 that substances such as copper sulphate, potassium hydrate, &c., 

 which possess such an intense affinity for water, should be 

 capable of existing in the anhydrous condition in the presence of 

 an unlimited amount of water. 



We know, moreover, that in a great number of cases — where 

 a dehydrated salt is placed in water — hydration does undeniably 

 precede dissolution,' and in such cases the salt can only exist in 

 the liquid in the uncombined state if the continued action of the 

 solvent is to decompose the hydrate which it has just formed. 

 The only two forces by which such a decomposition might be 

 supposed to be effected are (i) the attraction of the bulk of the 

 water present for the few molecules of water combined with the 

 salt ; (2) the attraction of this same bulk of water for the (anhy- 

 drous) salt molecules. On the one hand, however, it is absurd 

 to imagine that the mass of water molecules possess such a strong 

 atti action for the few contained in the hydrate as to decompose 

 this latter, or, even if they did, that they would ever have given 

 them up to the salt in the first instance ; and, on the other hand, 

 it is equally absurd to urge the intensity of the attraction of the 

 salt molecules for the water molecules as a reason for these two 

 parting company. 



Another general fact, which lends considerable support to the 

 view that the dissolution of a salt is due to the formation of 

 a hydrate, is, that those salts which coml/ine with water always 

 dissolve in that liquid, and, as a rule, the greater the energy with 

 which they do combine with it, the greater is their solubility. 



The thermal phenomena attending the act of dissolution point 

 incontestably to the same conclusion. When a dehydrated salt 

 (say MgSOj) is dissolved in water a considerable evolution of 

 heat occurs : and by the simplest experiment it can be established, 

 beyond any possibility of doubt, that all, or the greater portion 

 of this heat is due to the hydration of the salt. If the salt be 

 taken in the hydrated condition less heat is evolved, and, with- 

 out a single known exception, this evolution diminishes con- 

 tinuously as the salt taken is more and more highly hydrated ; 

 but even when taken in its most highly hydrated condition the 

 evolution of heat is in many cases still very c insiderable.'- Now, 

 unless we can reconcile ourselves to attribute the heat evolution 

 in this latter case to a cause entirely different from that which 

 exists in the other cases, — unless we are content to shut our eyes 

 to the proportionality between the heat evolved and the degree 

 of hydration of the salt taken, — we must admit that even with a 

 fully hydrated solid salt the heat evolved is due to further hydra- 

 tion ; that not only do hydrates exist in solution, but that they 

 are often of a higher orderthan the highest known in the solid 

 condition. 



Coming now to the other side of the question, we find many 

 general considerations, as well as special results, brought forward 

 against the hydrate theory of solution. The latter, however, 

 are for the most part, I consider, urged on mistaken notions, and 

 prove nothing pro or can. 



Thtis Dr. Nicol's study of the molecular volumes of salts in 

 solution shows that their volumes are entirely uninfluenced by 

 the presence or absence of water of crystallisation in the solid 

 salt ; that if any water is still combined with the salt when dis- 

 solved it acts in the same way, and is quite indistinguishable 

 from the rest of the solvent present. In so far as his conclusion 

 that these molecular volumes afford no evidence in support of the 

 existence of combined water, I entirely agree with Dr. Nicol ; 

 but in concluding that therefore no water is combined, he has 

 pushed his conclusions far beyond legitimate limits. The same 

 reasoning that leads to the belief that the water and the salt 

 bear no chemical relationship towards each other in solution 

 wotild hold equally good with reference to the radicles of which 

 the salts themselves are constituted, as Favre and Valson indi- 

 cated in 1875 [Comp/es Kcndiis, Ixxv. 1000). Each radicle 

 possesses its own specific volume entirely uninfluenced by the 



' Dr. Nicol (/'////. Mag. 1885.1. 453, and ii. 295) quotes experiments with 

 sodium sulphate in opposition to this view. He shows that the dehydrated 

 salt may dissolve in water under certain circumstances without any signs of 

 previous hydration. When it does so, however, it forms a supersaturated 

 solution, which is certainly very different from a normal solution, being, 

 according to Dr. Nicol's determination of the solubility, due to the extension 

 at lower temperatures of conditions which e.xist naturally only above 33° : 

 but when it dissolves to form a normal solution it is witli evident si^ns of 

 hydration. Whatever this may prove as to the supersaturated solution, it 

 certainly does not prove that the normal solution contains the anhydrous salt, 

 — rather the opposite. 



= Thus the "true" heat of dissolution of MgSO,?!!^© is -1-7000 cal., 

 and even this number is probably 1000 to 3000 cal. too low, as it contains no 

 allowance for the heat of fusion of the fVIgSOj molecule. (See Chem. Soc. 

 Trans. 1886, 279. ) 



