the Cohesion-Figures of Liquids. 431 



drogen rings in air, which I thought had not been correctly stated 

 in the books, when he informed me that Professor Rogers had, a 

 few years ago, sent him a copy of a paper on this part of my 

 subject. I lost no time in reading this paper in the American 

 Journal of Science for September 1858*, and was delighted at 

 the ingenious mode in which the subject had been treated. An 

 apparatus was constructed, from which rings of the vapour of 

 sal-ammoniac could be shot out into the air at pleasure, while 

 another apparatus let fall into water rings of water charged with 

 chromate of lead, or carbonate of lead, sulphate of lead, sulphate 

 of baryta, cobalt blue, or a dilute solution of sulphate of indigo. 

 Professor Rogers noticed that the rotation of the ring in air 

 " has always one direction, the inner circumference being carried 

 forwards, or, in the ordinary mode of the experiment, upwards, 

 and the outer in the reverse direction/' and that " the rotation 

 is due to the combined agency of the outward impulse and the 

 resistance which the sides of the issuing mass encounter from 

 the edge of the opening, and from the air into which it is im- 

 pelled. The former of these forces, due to the tension propa- 

 gated through the reservoir, must to some extent operate in 

 diverging directions, while the resistance acts in nearly opposite 

 ones. Thus at the outset there would be produced a reversion 

 or curling of the issuing cloud around the aperture, which, as 

 the action continued, would be developed into the spreading 

 volutes before described, and at length into the perfect and 

 rapidly revolving ring. The dilation of the ring in its ascent 

 would seem to be the natural result of the divergent character 

 of the impulse impressed upon the air as it passes from the 

 orifice.'" In the case of the phosphuretted hydrogen ring, Pro- 

 fessor Rogers insists upon " the chemical action of great in- 

 tensity that sets in between the included gas and the air." 



I admire Professor Rogers's paper so much, that I do not 

 .ike to criticize it ; but I cannot help thinking that his explana- 

 tions would be simplified- had he taken the case of a ring of 

 smoke ascending in air, or of a drop of a solution descending 

 in water, without embarrassing the subject with conditions de- 

 pending (1) on the form of the orifice, (2) the tension of 

 the enclosed gas, (3) the chemical action, and (4) the force 

 with which the liquid is projected. Take the case of a liquid 

 ring. The forces are (1) diffusion, which forms the ring; 

 (2) gravity, which causes it to sink. The resistance is friction, 

 retarding (1) the descent of the ring, (2) its diffusion. In the 

 case of a ring of smoke, the forces are precisely the same, only 

 gravity causes it to ascend, and friction retards the ascent. I 



* " On the Formation of Rotating Rings by Air and Liquids under 

 certain conditions of Discharge," by Prof. W. B. Rogers, p. 246. 



