332 



Mr. T. C. Porter 



on the 



and beautiful way the lowering o£ the boiling-point of water 

 under reduced pressure ; but there are some very curious phe- 

 nomena to be presently described, which are shown by the 

 column of steam, if the water is first stirred and then left to 

 come to rest, whilst the heating is continued. Just after 

 the stirrer has been removed, the appearance presented is 

 that recorded in fig. 1. 



The lengths o£ the multitude of curved lines, shown in 

 the original photographs near the bottom of the beaker, and 

 formed by the rotation of small stray bubbles, are an index 

 to the speed with which the water is rotating when the 

 duration of the exposure for the photograph is remembered ; 

 and in fig. 1 the rate of rotation is much higher than in the 

 subsequent figures, which are taken at later stages. In fig. 1 

 there is a markedly concave surface to the water in the 

 beaker, and the column "of steam is practically continuous 

 from base to summit where it joins the air. This phase 

 lasts about a minute, when the water has been stirred as 

 rapidly as is possible by hand, and then it will be noticed 

 that pulsations set in : at first these are feeble, and succeed, 

 each other with great rapidity; but their period rather rapidly 

 lengthens till it may last four seconds or more, and at the same 

 time they become more and more violent. 



The course of events during a single pulsation is as follows: — 

 1st phase, the surface-curve of the water flattens, and in the 

 later stages of the experiment the curvature disappears ; whilst, 

 so far as can be judged by eye and from the photographs, 

 at the instant when the surface of the water is most nearly 

 level, or just before it, a column of steam springs up with 

 great rapidity from the base of the beaker to the surface of 

 the water, heaving this up in its central portion, and in the 

 later stages of the experiment often causing the ejection of 

 water from the beaker. This phase is shown in fig. 2, where 

 the reversal of the surface curvature is 

 very evident. Immediately after the 

 eruption of the steam, and whilst the 

 steam-column still stretches from the 

 base of the beaker to the surface of the 

 water, follows the 2nd phase. The 

 steam-column seems to condense and 

 breaks up, leaving only a few small 

 bubbles, which either hang stationary 

 or move downwards in the liquid ; 

 whilst if the water has dust in it the 

 motion of the dust particles shows that 

 a curious kind of annular wave, concentric with the steam- 



Fig. 2. 



