6 AMCEBA LESS. 



There is another important property of proteids which is 

 tested by the instrument called a dialyser. This consists 

 essentially of a shallow vessel, the bottom of which is made 

 of bladder, or vegetable parchment^jat, some other organic 

 (animal or vegetable) membrane. If a solution of sugar or of 

 salt is placed in a dialyser and the instrument floated in a 

 larger vessel of distilled water, it will be found after a time that 

 some of the sugar or salt has passed from the dialyser into 

 the outer vessel through the membrane. On the other hand, 

 if a solution of white of egg is placed in the dialyser no 

 such transference to the outer vessel will take place. 



The dialyser thus allows us to divide substances into 

 two classes : crystalloids so called because most of them, 

 like salt and sugar, are capable of existing in the form of 

 crystals which, in the state of solution, will diffuse through 

 an organic membrane ; and colloids or glue-like substances 

 which will not diffuse. Protoplasm, like the proteids of 

 which it is largely composed, is a colloid, that is, is non- 

 diffusible. 



Another character of proteids is their instability. A 

 lump of salt or of sugar, a piece of wood or of chalk, may 

 be preserved unaltered for any length of time, but a proteid 

 if left to itself very soon begins to decompose ; it acquires 

 an offensive odour, and breaks up into simpler and simpler 

 compounds, the most important of which are water (H 2 O), 

 carbon dioxide or carbonic acid (CO 2 ), ammonia (NH 3 ), 

 and sulphuretted hydrogen (H^S) 1 . In this character of 

 instability or readiness to decompose protoplasm notoriously 

 agrees with its constituent proteids ; any dead organism will, 



1 For a more detailed account of the phenomena of putrefaction see 

 Lesson VIII., in which it will be seen that the above statement as to 

 the instability of (dead) proteids requires qualification ; as a matter of 

 fact they onty decompose in the presence of living Bacteria. 



