408 



MR, T, GBAHAM ON THE ABSORPTION AND 



The pneumatic instrument of Dr. Speengel (fig. 1) is peculiarly applicable to researches 

 of the present kind. Indeed without the use of his invention some parts of the inquiry 

 would have been practically impossible*. The in- p-jg j^ 



strument was originally offered by the inventor as the 

 means of producing a vacuum, or as an air-pump. 

 But by bending the lower end of the straight fall- 

 tube, the instrument may be further made to deliver 

 gas into a receiver, and be used with advantage as 

 the means of transferring small volumes of gas from 

 one vessel to another. 



While the mercury in the funnel A is allowed to 

 flow downward into the barometer-tube CB, of 2 J 

 millims. in diameter, by relaxing the clamp upon the 

 adapter tube of rubber at C, a connexion is also 

 made with the close receiver to be exhausted, such 

 as an air-tight bag E, by means of the branch tube x. 

 The air inE, gaining access to the Torricellian vacuum, 

 is swept on by the falling mercury, and delivered 

 below into the small gas-receiver R, previously filled 

 with mercury and inverted over mercury in the 

 mortar B below. The principal difficulty in obtaining 

 a good vacuum in E by means of this apparatus 

 arises from the necessity of joining the glass tubes in 

 more than one place by means of adapter tubes of 

 rubber. The directions given by Dr. Speengel on 

 this point require to be closely followed : — " The connexions between the glass tubes are 

 made of well-fitting black vulcanized caoutchouc tubing, sold under the name of French 

 tubing. This is free from metallic oxides, which render the tubing porous. Besides 

 this all these joints are bound with coils of copper wire, which is easily accomplished 

 with a pair of pliers." The joints should also be coated with gutta percha liquefied by 

 heat, or with fused rubber. An exhausting-syiinge, or air-pump, may often be used with 

 advantage to begin the exhaustion, and to vpithdraw the greater bulk of the air, if the 

 receiver is large, the Sprengel tube being reseiTed to complete the exhaustion. The 

 vacuum appears to be as perfect as can be formed in a barometer-tube filled with unboiled 

 mercury, and to come within 1 millim. of the barometric gauge. 



The following modifications of the experiment exhibit the dialytic action of caout- 

 chouc in its various forms. 



* Researches on the Vacuum, by Hermann Speengel, Ph.D., Chemical Society's Journal, ser. 2, vol. iii. p. 9 

 (January 1865). 



