234 Proceedings of the British Association. 



He then tried the effect of interposing a sheet of writing paper, 

 without the water-mark and of uniform texture. The result of this 

 experiment fully answered his expectations. The diffusion of the 

 light thus occasioned shaded off, as it were, all the sharp lines and 

 points, and gave a high degree of softness to the picture. The effect 

 was even improved by interposing two sheets of clean paper ; and, 

 with a very bright meridian sun, he found that three sheets may be 

 used with advantage. A similar effect may be obtained, in a smaller 

 degree, by placing the back of the negative upon the positive paper, 

 so as to cause the light to traverse the thickness of the negative, and 

 this may be combined with one or more sheets of clean paper. This, 

 of course, will be appropriate only with portraits ; and it has the ad- 

 vantage (sometimes required) of making the figure look another way. 

 To those who see the experiments above described for the first time, 

 the effect is almost magical ; and when the negative is removed, we 

 see only a blank sheet of white paper ; and our surprise is very great 

 when, upon lifting this sheet, we discover beneath it a perfect picture, 

 which seems as it were to have passed through the opaque and 

 impervious screen. Sir D. Brewster exhibited specimens of por- 

 traits produced in this manner, and also specimens produced by the 

 transmission of light through two perfectly coincident negatives of dif- 

 ferent degrees of strength ; together with specimens of positives, 

 produced by placing the positive paper between two perfectly coinci- 

 dent negatives, and acted upon by light incident on both sides of the 

 picture. Sir D. Brewster mentioned some unexpected theoritical 

 results, which these experiments indicated, but which required further 

 investigation. 



Saturday. 

 Section B.— CHEMISTRY AND MINERALOGY. 

 c On a New Property of Gases,' by Prof. Graham. — After explaining 

 the law which regulated the diffusion of gases through porous bodies, 

 and stating the fact, that the lighter gases diffused themselves much 

 more speedily than the more dense ones, — that their diffusion was 

 equal to the square of their densities, — he proceeded to relate his 

 experiments on the passage of gases into a vacuum. To this passage 

 the term Effusion has been applied. The velocity of air being 1 ; the 



