PROCEEDINGS OF SOCIETIES. 45 



It was stated that Herr Moller prepared slides with 100 as well as 

 those with 80 specimens, aud was about to introduce similar slides of 

 Echinoidea, Holothuridaj, &c. "With difficult objects like diatoms, the 

 advantage of having a well-assorted series in one slide with the names 

 attached was obviously great, and would no doubt be appreciated by 

 all students as well as by collectors of microscopical curiosities. 



Mr, Slack also said that he was asked at the last meeting if he had 

 seen silica solution in the milky condition described by Mr. Eead.* 

 Since that meeting he had received from Mr. Eead a specimen, the 

 whole of which was milky, and the question was whether the particles 

 could be seen. He had examined some of the fluid with various 

 powers and imder diiferent illuminations, but had not succeeded in 

 seeing the particles. If a drop or a thick film were put upon a glass 

 slide and evaporated, the result was a film of the silica with the cracks 

 in it; but if a thin smear only were put on the glass, then they got 

 thousands of spherical particles. The deposit was probably a hydrate 

 of silica, and he thought it jjossible that particles ought to be seen if 

 the power employed was good enough. He had sent some of the 

 solution to Dr. Anthony, and it appeared that he did see numerous 

 aprticles with a ^-inch objective by Eoss, in a dark room with a beam of 

 light let in through a hole in a shutter, but he was unable to see 

 them with a higher power. This reminded him of some remarks by 

 Professor Tyndall, who had stated that if a little mastic dissolved in 

 alcohol was added to water in sufficient quantity to produce a sky-blue 

 effect, the particles could not be seen by any known microscopical 

 powers ; but if a small drop of it were taken and put into a little 

 water, then they could be seen. "Why was it that they could be seen 

 in the small quantity, and yet not in the large ? He had not yet 

 material enough to write a paper on the subject, but thought ho 

 might mention it as being one of interest. 



The President believed that it was entirely a matter of molecular 

 aggregation. He remembered that some years ago the late Professor 

 Faraday gave him a bottle of liquid containing gold in a minute state 

 of subdivision, so that the fluid appeared of a rose colour. He sub- 

 mitted it to examination with the highest powers, and illuminated it 

 in all ways, but was not able to trace any sign of the particles. It 

 remained so for many years, and at the present time there was a 

 quantity of the minute particles of gold at the bottom of the bottle as 

 a sediment ; but if shaken up, they remained merely mechanically 

 suspended in the liquid, and could be seen as molecules, which settled 

 again to the bottom in the course of a short time. In dissolving a 

 substance it might be that its molecules were so widely separated that 

 they passed beyond each other's sphere of attraction, and that if placed 

 in a small quantity of fluid they might be brought nearer, and thus 

 within the sjihere of mutual attraction. He believed another instance 

 was furnished by the preparation of chromate of lead which was used 

 for injections. These injections it was found could only be made 

 with a solution of the precipitate which was freshly prej)ared ; if it 

 were allowed to remain a long time, the molecules became larger, and 

 the finest capillaries could not any longer be injected with them, 

 * 'Monthly Microscopical Journal,' Jnne, 1874, p. 272. 



