51 



because there was no reason why diatoms should not be found 

 in the Silurian rocks, but, on the contrary, it might rather 

 be expected that they would. 



Mr. Karop thought that the need for care which Mr. Newton 

 had insisted upon could not be too strongly enforced in making 

 any investigations in this direction; everything used should be 

 perfectly free from suspicion, and new pipettes must be used, 

 because the diatoms would cling to anything which once 

 contained them for an indefinite time. 



The President said he remembered seeing some curious 

 diatoms in some kind of rock which was sent to them by Mr. 

 Shrubsole. 



Mr. Karop said they were common enough in the Tertiary 

 strata, but had not until quite recently been found in strata as 

 old as the Silurian. 



Mr. E. T. Newton said it was generally accepted that the 

 higher the grade of an animal the shorter the distance it went 

 back in time ; and it was of course also true, on the con- 

 trary, that the lower the grade the farther back it was found to 

 extend; if this were true they might expect to find diatoms 

 in the Silurian. 



Mr. Karop thought it was understood that their age was 

 great, but being formed of colloid silica, which was soluble in 

 alkaline water, it seemed rather doubtful if they would be able 

 to persist in the manner expected. 



Mr. Newton said that the evidences of their existence would 

 not necessarily be destroyed even if they were dissolved, because 

 they would leave their impress in the rocks, and this might 

 be filled up with other matter. 



Mr. Morland could quite corroborate what Mr. Karop had 

 said as to the way diatoms had of clinging to tubes, etc. No 

 one could ever be sure that a tube was quite clear from them, 

 even after repeated washing and wiping. 



The President made some remarks upon the subject of 

 coloured screens for use with the microscope, and pointed out 

 the advantages to be gained thereby. These could be of a 

 variety of tints, but all would not be found to work equally well 

 for all purposes. Colonel Woodward's screens were made with 

 a solution of ammonio-sulphate of copper, whilst his own were 

 of cobalt blue glass, and Mr. C. Haughton Gill used a Zettnow 



