272 PBOCEEDINGS OF SOCIETIES. 



increase in size as it was made red hot. It just struck him that this 

 appearance might be the result of the greater amount of light passing 

 through these objects, and producing irradiation with lower powers. 



Mr. W. T. Kead said that he had been for some time much 

 interested in silica, but he had always obtained it for purposes of 

 experiment from a silicate of potash which he neutralized by various 

 acids. If he took a solution of silicate of soda and added hydro- 

 chloric acid, then this rushing together took place ; but if a weaker 

 solution and a weaker acid were used, such as sulphurous or even 

 carbonic acid, they would find that after a time, say in a day or two, 

 the liquid, instead of resolving itself into a jelly, would become milky 

 or cloudy in appearance, which he thought was the result of innu- 

 merable spherules being deposited, but yet remaining suspended in 

 the fluid. This condition he regarded as intermediate between silica 

 in solution and silica in a gelatinous form. 



Mr. Slack said that he had never examined them exactly in that 

 way ; but he thought if a milkiness were obtained, there would be a 

 chance of seeing the particles. 



Mr. Eead said that if a solution of silicate of soda was obtained, 

 and it was neutralized with hydrochloric acid and then dialyzed, they 

 would obtain silica simply in solution. Some of this solution had been 

 obtained which had remained liquid for a great period of time, and he 

 thought that this was due to its having been accurately pure ; but it 

 was easily disturbed, and a very little disturbance would cause it to 

 gelatinize. If it were taken of a certain density, it would be found 

 to assume a cloudy condition preliminary to the particles rushing 

 together and forming a jelly. This milkiness was increased when the 

 strength of the solution was reduced. 



Mr. Slack thought there was a great tendency to coalesce without 

 assuming a spherical form. He should much like to see it in the state 

 described by Mr. Read. 



Mr. Eead said he should have great pleasure in forwarding a 

 sample of it. He had a practical object in view in making his experi- 

 ments ; and he believed that silica was a substance which would 

 presently become of great interest. He had already foimd out 

 chemical properties of a very remarkable character ; but he found that 

 silica in solution was very apt to rush into the amorphous form. 



A paper by Dr. Pigott, " On the Use of Black Shadow Markings 

 and on a Black Shadow Illuminator," was (owing to the lateness of the 

 hour) taken as read, the apparatus described being placed upon the 

 table for the inspection of the meeting. The paper will be found 

 printed at p. 246. 



Mr. Wenham thought that it was an extension of the principle 

 which had already been brought under their notice. He had himself 

 used tin-foil above the objects, and the condensing apj^aratus under the 

 slide. 



Mr. Frank Crisp observed that Dr. Matthews brought out a plan 

 for tilting the condenser in the same way, and had several of them 

 made by Mr. Swift. 



The gentleman elected on the 1st of April was Robert Home, 

 Esq., not Herbert Horn, as printed in the last number. 



