54 Prof. J. P. Cooke on Lecture Experiments . 



faucet and the gas burnt as it is forced out through the over- 

 flow-tube. 



We have now proved, first, that hydrochloric acid is composed 

 of hydrogen and chlorine, secondly, that these gases are present 

 in equal proportions by volume; and in order to complete our 

 demonstration of the constitution of this typical compound, we 

 have only to prove that when these equal volumes unite to form 

 HC1 there is no condensation. This we may do either synthe- 

 tically or analytically. 



The synthetical experiment is made by filling the absorption- 

 tube as before with the mixed gases obtained by the electrolysis 

 of hydrochloric acid, taking great care, as also in the previous 

 experiment, that the tube shall be perfectly dry. It will of 

 course be necessary to have two of these tubes if both experi- 

 ments are to be made in the same lecture. The tube thus filled 

 is to be exposed for some time to bright diffused sunlight, and 

 afterwards directly to the sunbeam until perfect combination 

 results. If the glass is quite thick and the rubber stopper is 

 forced in very tight, the gases may even be exploded in the 

 tube without danger, although this form of the experiment is 

 always attended with some risk. When the gases are in either 

 way thoroughly combined, the end of the exit-tube is first 

 opened under mercury, in order to prove that no condensation 

 has taken place, and afterwards under water coloured blue by 

 litmus, when a very rapid absorption takes place, and the red- 

 dening of the water will indicate that HC1 has in fact been 

 formed. 



The analytical experiment is made by decomposing HC1 gas 

 by means of sodium-amalgam. For this purpose the carefully 

 dried gas is first collected over mercury in a perfectly dry tube. 

 This tube we assume is graduated, or at least divided into two 

 parts of equal capacity. The tube full of gas is now slipped 

 over the mouth of a rubber cap, which has been previously 

 filled with the amalgam (see fig. 5, left side of the woodcut), and 

 sunk in the mercury-trough until the liquid is just level with 

 the brim. This cap, which is easily made from the " stopper- 

 cord," should be capable of holding 10 or 20 cubic centims. 

 of the amalgam, and should tightly clasp the tube. Moreover, 

 for this purpose^ the solid amalgam above described should be 

 reduced with mercury to the consistency of a thin paste. The 

 amalgam is now to be shaken up in the tube for several minutes 

 until the decomposition is complete, and then the mouth of the 

 tube may be opened under mercury. The liquid will rise to 

 supply the place of the chlorine which has been absorbed; and 

 on sinking the tube in the mercury-trough until the level is the 

 game within as without, it will appear that exactly one-half of 



