176 



Mr. Golding asked if there was not some danger of Podura Scales falling about 

 in so great a space as that contained by the india-rubber ring ? 



Mr. Locke said that he had found in practice that scales once placed upon 

 the glass cover would adhere to it and remain there. Of course if there were any 

 contact between the cover and the slide the scales would be damaged, and he 

 thought it impracticable to get a cell of exactly the depth required. 



Mr. Greenish inquired what was the object in heating the cell ; why was it 

 better then than otherwise ? 



Mr. Locke said that his object in heating it was to make it adhesive ; in its 

 ordinary condition it would not adhere by itself, and, if fixed on with varnish, 

 that would evaporate inside as well as outside the cell. 



Mr. Greenish thought it was quite possible that by decomposing the india- 

 rubber in that way an element was introduced which might prove very 

 inconvenient. 



Mr. Burr suggested that the vulcanized india-rubber contained sulphur. 



Mr. Golding thought it probable that the sulphur was got rid of by heating the 

 rings in the manner described. 



Mr. Greenish said that at a former discussion upon a similar subject a member 

 had stated that he had found the sulphur to be very injurious. 



Mr. Locke said that on one occasion being too anxious to get the cell perfectly 

 dry he had heated and softened the ring too much, and found that on cooling 

 it collapsed from the pressure of the atmosphere. He thought this conclusively 

 showed how perfectly air tight it must have been. 



The President said it showed that sufficient heat was applied to drive out all 

 the air from the cell. 



Mr. B. D. Jackson thought there was one important question which had not 

 been touched upon, and that was the permanent nature of the ring. He also 

 thought that the large space between the object and the condenser, owing to the 

 thickness of the ring, would act prejudicially against proper illumination of 

 diatoms or scales. 



Mr. Locke said that it might possibly be so, but it should be remembered that 

 in mounting objects of extreme tenuity they were always placed on the cover and 

 not on the slide. 



Mr. Jackson remarked that Ross's io condenser certainly would not work up 

 near enough to the cover of the slide which had been handed round for 

 inspection. 



Mr. Locke thought that in this case they must, as in all others, do the best 

 they could. 



Mr. T. C. White suggested the employment of thin sheet gutta-percha instead 

 of india-rubber. 



Dr. Gray believed that a piece of sheet gutta-percha would be found to answer 

 the purpose very well ; it could be obtained of any thickness. Dr. Tulk re- 

 commended it to him, and he found that it made a perfectly air tight cell. For 

 Buch things as scales nothing could be better than a cell made of thin sheet 

 gutta-percha fixed to the slide with a little electrical cement run round the edge ; 

 it would run under by capillary attraction, and formed a capital cement for the 

 purpose. 



Mr. Greenish said it should be borne in mind, with regard to gutta-percha and 

 india-rubber, that both underwent decomposition spontaneously from exposure 

 to the air. 



Mr. Locke thought that if the cell was air tight, and a coat of varnish was ap- 



