m THE PRESEA^CE OE DUST-FREE AIR AHD OTHER GASES. 
277 
The machine used in nearly all the experiments described below was made from 
moderately tbick-walled tubing, having an internal diameter of 2 centims. The 
expansion chamber A was between 4 and 5 centims. long, and bad a capacity, when 
the piston was in its lowest position, of about 15 cub. centims. The cylindrical part 
of the piston was about 3 centims. in length. 
The time required for the expansion to be completed must be very short. For the 
distance travelled by the piston was never so great as 2 centims., even with the 
greatest expansions used. To support the weight of the piston alone, required an 
excess of pressure below of 1 millim. of mercury. Now the driving pressure, even 
when the expansion was almost completed, was probably never less than 100 times 
as great as this. With the largest expansions used, when the piston had still to 
travel less than 2 centims., the initial driving pressure was more than an atmosphere, 
that is, about 760 times that required to balance the "weight of the piston. This is 
on the assumption of an exceedingly rapid fall of pressure in the space below the 
piston when the plug G is driven up. If we make this assumption, the force driving 
the piston is always some hundreds of times its weight, and its initial acceleration 
some hundreds of time “ g.” Even an average acceleration of lOOq would enable the 
2 centims. to be travelled in about of a second. 
Although the time taken may actually be considerably greater than this, these 
considerations are sufficient to show that it is likely to be exceedingly short. 
Further, the piston must be moving with constantly increasing velocity till brought 
to a sudden stop at the constriction into which it fits. 
The fact that contact takes place simultaneously over a considerable area, probably 
saves the tube from being broken by the blow it receives from the piston. The film 
of water which covers both surfaces, doubtless helps to brea,k the shock. On two 
occasions, a machine was made only slightly larger than the one whose dimensions 
are given above, but in each case it was shivered by the imi^act almost the first time 
the piston was allowed to fly. More than one machine was useless, owing to the 
piston being driven so tightly home when the expansion was made, that all efforts 
failed to release it. To avoid this latter defect, it was found necessary to make the 
constriction a very sudden one. Great care has also to be taken to make it perfectly 
symmetrical; otherwise there is almost certain to be a space left between the piston 
and some part of the vnll of the tube just above the constriction. In this, air-bubbles 
are apt to be entangled when the piston flies into the constriction. These may work 
their way up into A, and in addition they cause a splashing of the water by their 
momentary compression and subsec^uent expansion. 
A supply of water for the lubrication of the piston is stored in the vessel B. The 
Space over the mercury is completely filled with water up to the stopcocks Tg and Tg, 
which remain closed throughout any series of experiments. By fixing the mercury 
reservoir E-i at a sufficient height, the water in B is kept at a pressure high enough 
to drive it into A when the tap T, is opened. In this way the water is preserved 
