Peculiar Properties of Glass. 51 



Both were placed in hydrofluoric acid, with the result that 

 the acid dissolved away the surface in the first, whilst, 

 in dissolving away the surface from the bottom the whole 

 drop became disintegrated and was found in small pieces. 



The experiment was repeated, and this time both top 

 and bottom surfaces were removed respectively to a depth 

 of about -iVth of an inch and the drops remained intact. 

 When the tail of the first with the top surface removed was 

 broken off, the drop remained intact, and it was only when 

 the glass was broken near the point A in a vice that the 

 bottom part became disintegrated. 



In the second drop, when the lower surface only was 

 removed, the breaking of the tail burst the whole drop, but 

 the lower part broke into much larger pieces than it would 

 have done if the surface had not been removed. 



According to Robert Hooke you may grind away the 

 bottom of the drop without producing disintegration, but if 

 this be attempted from the point downwards the drop in- 

 variably bursts. From the above experiments one is led to 

 believe that the drop might be ground from either end if 

 the necessary care were taken, which would no doubt require 

 to be much greater from the point downwards than from 

 the bottom upwards. 



The explanation of the bubbles in the drops seems to be 

 that there are very minute bubbles of air in the glass, which 

 form nuclei for the formation of the vacuous spaces, and 

 where none of these nuclei exist the drop appears to form 

 as a solid transparent mass under greater tension than those 

 in which the bubbles have formed ; but the curious thing is 

 that whilst the Rupert's drops containing bubbles had in- 

 creased in volume over 2 per cent, those free from bubbles 

 had only increased by about ^ per cent. One would 

 suppose that if a drop of molten glass were thrown into cold 

 water its external surface would be solidified at once and 

 that, whether or no, bubbles formed afterwards in the centre 



