306 FRAGMENTS OF SCIENCE. 



been carried mechanically into the experimental tube. Precautions were 

 therefore taken to prevent any such transfer. They produced little or no 

 mitigation. I did not imagine at the time that the dust of the external 

 air could find such free pa'ssage through the caustic potash and the sul- 

 phuric-acid tubes. But the motes really came from without. They also 

 passed with freedom through a variety of ethers and alcohols. In fact, 

 it requires long-continued action on the part of an acid first to wet the 

 motes and afterward to destroy them. By carefully passing the air 

 through the flame of a spirit-lamp or through a platinum tube heated to 

 bright redness, the floating matter was sensibly destroyed. It was there- 

 fore combustible, in other words, organic matter. I tried to intercept it 

 by a large respirator of cotton-wool. Close pressure was necessary to 

 render the wool effective. A plug of the wool rammed pretty tightly into 

 the tube through which the air passed was finally found competent to 

 hold back the motes. They appeared from time to time afterward and 

 gave me much trouble ; but they were invariably traced in the end to 

 some defect in the purifying apparatus to some crack or flaw in the 

 sealing-wax employed to render the tubes air-tight. Thus through proper 

 caie, but not without a great deal of searching out of disturbances, the 

 experimental tube, even when filled vdth pure air or vapor, contains 

 nothing competent to scatter the light. The space within it has the as- 

 pect of an absolute vacuum. 



An experimental tube in this condition I call optically empty. 



The simple apparatus employed in these experiments will be at once 

 understood by reference to the figure on page 307. S S' is the glass ex- 

 perimental tube which has varied in length from 1 to 5 feet, and which may 

 be from 2 to 3 inches in diameter. From the end S the pipe p p' passes to 

 an air-pump. Connected with the other end S' we have the flask F, con- 

 taining the liquid whose vapor is to be examined ; then follows a U-tube, 

 T, filled with fragments of clean glass wetted with sulphuric acid ; then 

 a second U-tube, T', containing fragments of marble wetted with caustic 

 potash ; and finally a narrow straight tube, 1 ', containing a tolerably 

 tightly-fitting plug of cotton-wool. To save the air-pump gauge from the 

 attack of such vapors as act on mercury, as also to facilitate observa- 

 tion, a separate barometer tube was employed. 



Through the cork which stops the flask F two glass tubes, a and 5, pass 

 air-tight. The tube a ends immediately under the cork ; the tube b, on 

 the contrary, descends to the bottom of the flask and dips into the liquid. 

 The end of the tube b is drawn out so as to render very small the orifice 

 through which the air escapes into the liquid". 



The experimental tube S S' being exhausted, a cock at the end S' is 



