four curved slots iu it, into which the pins tit, and it is so 

 arranged that as the brass circle is turned in one direction, 

 the four pieces whicli constitute the diapliragm are brought 

 nearer to the centre of the condenser, at exactly the same 

 rate, the aperture becoming gradually smaller, while if the 

 circle be turned in the opposite direction the diaphragm 

 pieces move away from the centre, and the aperture becomes 

 larger. This movement can of course be made while we are 

 observing, and thus the best possible effect be secured. The 

 only disadvantage of this arrangement is, that there is a 

 slight change of form in the aperture, as it is contracted, but 

 for most purposes this is unimportant. Now this method of 

 making four points simultaneously approach the centre of a 

 circle had been known to mechanics for years before any one 

 thought of applying it to this particular purpose. Free 

 communication between optical and mechanical students 

 might have given us this instrument much sooner. Again, 

 would anyone look to obtain an illustration of the motion of 

 air in storms while attempting to improve a gas burner? 

 Yet such has been done ; here is a burner j ust short of 

 perfection, the supply of air in the centre not being quite 

 sufficient to burn all the carbon, and the ignited particles of 

 carbon show us the movement of the currents of air, which, 

 rushing into the upward current of the burner from all sides, 

 produce a circular movement illustrating to the eye the 

 movement of air in a storm. One more illustration and I 

 have done. There exists in the young bark of the willow and 

 the poplar a substance called salicin. This white crystaline 

 body which, chemically speaking, is a glucoside of saligenin, 

 a substance having the composition C. Hg O^. When this 

 is treated with oxidizing agents it yields water and salicylic 

 aldehyde (C, Hg 0, + O = C^ Hg 0^ + H^ 0), a body 

 bearing the same relation to saligenin that ordinary alde- 

 hyde does to alcohol. If we distil salicin with a mixture, 

 of sulphuric acid and bichromate of potassium, an operation 

 which has been going on all this time in the retort before me, 

 we obtain this body, which is you see, a yellowish oil, and 



