90 Mr. C. Tomlinson on some Phenomena 



practice of distillation fresh charcoal would be taken in each 

 operation. 



In the course of numerous operations with successive charges 

 of wood-spirit and other liquids, I had frequent opportunities 

 of noticing a fact which was pointed out in my paper read before 

 the Royal Society* — namely, that if the thermometer contained 

 in a liquid at, or near, or above the boiling-point be taken out, 

 waved in the air, and then reinserted, it produces a fresh burst 

 of vapour. M. Gernez criticises this experiment, apparently 

 without repeating it, so impressed is he that the thermometer 

 thus exposed takes iip a film of air, which, with him, is the only 

 active nucleus. But is not this a little contradictory? He 

 labours to prove that a clean surface cannot contract a film of 

 air; the thermometer in the above experiments presents a clean 

 surface, and the momentary exposure is not sufficient to convert 

 it into a dirty one. But it is sufficient to allow the thermometer 

 to pick up some particle of dust, a mote or a speck, which acts 

 as a nucleus. The thermometer used by me has its scale en- 

 closed within a glass tube, the junction of which with the elon- 

 gated bulb forms a somewhat deeply indented neck, which seems 

 to be peculiarly favourable for receiving and retaining a nuclear 

 speck ; so that in taking out the thermometer for the purpose 

 of recharging the retort, it occasionally picked up a nucleus from 

 the air ; and on restoring the thermometer to its place, the ope- 

 ration was sometimes disturbed by the activity of this nuclear 

 point, from which issued an inverted cone of bubbles of vapour, 

 often accompanied by a low musical note. On some occasions 

 the speck would be caught up by the bulb, or two or three 

 specks at different parts of its surface, each originating a cone of 

 bubbles. 



In my paper in the ' Proceedings ' I pointed out that the 

 retort and other vessels used in the boiling and distillation of 

 liquids frequently contain nuclear points which greatly assist the 

 operation, and tend to save the vessel from destruction. When 

 the mode of heating is by means of a hot bath, these nuclear 

 points appear to become active long before the liquid has at- 

 tained the boiling-point. Thus in heating wood-spirit, which 

 boils at 148°, a nuclear point in the retort often becomes active 

 at 90°. The action of a bath on the belly of a retort of thin 

 glass appears to be this : — The sides of the retort rapidly attain 

 the temperature of the bath, which we may suppose to be 

 at about 200°, while the liquid in the centre in contact with the 

 bulb of the thermometer is only at 90°. A thin layer of liquid 

 is everywhere repelled from the inner surface of the belly of the 

 retort and rises to the surface of the liquid, to be replaced by 

 * Proc. Roy. Soc. 1869, vol. xvii. p. 240. 



