454 CHEMICAL DISCOVERY AND INVENTION 



observed since 1898, and the reversibility of enzymic action is 

 now well recognised. 



The mechanism of enzyme action has received a great deal of 

 attention. How do these complex substances do their work ? 

 That in so many cases they are selective in their attitude toward 

 the carbohydrate, protein, or other substance with which they 

 are in contact suggested to Emil Fischer long ago the analogy 

 of the lock and key. When an enzyme finds itself in the presence 

 of two glucoses, for example, having the same composition, mole- 

 cular weight, and general character, but differing from each 

 other only in " configuration " as indicated by their optical 

 properties, the conclusion seems irresistible that the enzyme is able 

 to fit itself into the body of the one and not into that of the other. 

 This seems to imply that the preliminary to action is a state of 

 union between the enzyme and the " substrate," and the only 

 remaining question is whether this combination is of a chemical 

 nature, or a physical or mechanical nature. In the former case 

 definite proportions would be expected to interact or combine. 

 Evidence in this direction is, however, unsatisfactory. 



In the latter case the phenomena of " adsorption " are referred 

 to in which the amount of combination is dependent, as in the 

 case of other colloids, wholly on the extent of surface. Surface 

 combination is in many cases of a very intimate nature and quite 

 as difficult to dissociate as many a true chemical compound. 



Mention has been made in previous chapters of the tenacity 

 with which air adheres to the surface of glass, of the withdrawal 

 from solution of various substances, such as iodine, bromine, and 

 organic colouring matters by contact with charcoal, of the 

 staining of fibres of cotton, wool, hair, or silk, all colloids, by 

 dyes. 



That this adsorption is in many cases selective is shown by 

 the following facts. If a sheet of fine filter paper is wetted with 

 a drop of solution of a salt of silver, lead, or mercury moderately 

 concentrated, and the spot after a few minutes is exposed to 

 contact with sulphuretted hydrogen gas, a dark stain due to the 

 formation of the sulphide of the metal is produced in the centre 

 which is surrounded by a wide ring of pure water. To produce a 

 corresponding effect with a solution of copper, nickel, or cobalt, 

 a much more dilute solution must be used. On the other hand, a 

 solution of a cadmium salt, even very dilute, when exposed to 

 the gas gives a yellow stain of sulphide which extends quite to 



