TBANSACTIONS OF SECTION A. 661 



9. Proof that Density of Illumination does not affect Absorption. 

 By W. Peddie, D.Sc. 



When parallel rays of light pass through a medium which exercises no 

 absorptive influence, the intensity of the light is not diminished as it proceeds. 

 When the medium is absorptive, it is assumed as a basis for the theory of radiation 

 that if the intensity of the light be reduced in a given ratio on passage through a 

 given thickness of the medium, it will be further reduced in the same ratio on 

 passage throagh an additional equal thickness. There is strong indirect evidence 

 that this assumption is correct. Professor Stokes suggested long ago a direct 

 method of testing the point. In his method the light is reflected at nearly 

 perpendicular incidence from the surface of glass, and two equal portions of the 

 absorbing medium are used — one being placed in the course of one half of the 

 incident light, the other in the course of the other half after reflection. Assuming 

 that reflection is not aflected by intensity, we thus get a direct test of the question. 

 The author has used diverging light passing through two double-image prisms, a 

 plate of quartz being interposed between them, and a lens being used for the 

 purpose of proper definition. By rotation of one diprism, and by varying the 

 thickness of the quartz, we can match closely any colour of the absorbing medium, 

 and also get the complementary colour. One portion of the absorbing medium is 

 inserted in the course of one ray, the other in the course of the similarly coloured 

 ray, but at a diflerent distance from the point of divergence. Surface-coloured glass 

 was used in the experiments described. No difl'erence of absorption was evident, 

 although the intensity varied from one to 1,000, and a change of intensity alone 

 (unaided by colour comparison) of one part in 100 could be detected. 



10. Physical Conditions of the Waters of the English Channel. 

 By H. N. Dickson, F.B.S.E. 



Careful determinations of the value of Dittmar's ' D ' for numerous samples 

 obtained at different seasons of the year show that the waters of the English 

 Channel are uniform and constant in composition, at least as far east as the Isle of 

 Wight. The mean value of D was 1'4550, a result in exact agreement with that 

 found by Dr. Gibson for water collected in the Moray Firth, which he believed 

 had come from the Atlantic. 



The normal distribution of temperatua'e seems to be that in summer the coldest 

 water, and in winter the warmest, is found in mid-channel — the difl'erence 

 decreasing in summer and increasing in winter as we go eastward. Excluding a 

 thin surface layer, the temperature is nearly uniform at all depths. 



The principal exception to this rule occurs in the west of Start Bay, where a 

 mass of water, cut ott' from the general tidal circulation, is subject to peculiar 

 temperature conditions through the want of mechanical mixing. 



11. On the Analysis and Synthesis of Colour. By J. W. Lovibond. 



12. On the General Laws of Energetics. By Professor W. Ostwald. 



The mechanical law of the virtual velocities, or, more properly, of the virtual 

 work, is only a special case of a much more general law, holding good for all 

 forms of energy except radiant energy. The condition of equilibrium of every 

 energetic system can be found by summing up the virtual or correlative variations 

 of the various forms of energy and putting the sum equal to zero. In order to 

 express these virtual variations each energy must be written as a product of two 

 factors — its capacity and its intensity. 



As this calculus leads to the same results as those obtained up to the present 

 by the application of the second law of thermodynamics, it is possible to prove 

 that this second law is another case of the same general law of energetics, which 



