48 SCIENCE PRIMERS, [§ iv. 



fluid ? This is rather a difficult question to answer. 

 When blood is shed from the warm body it soon 

 gets cool. But it does not clot and become solid 

 because it gets cool, as ordinary jelly does. If you 

 keep it from getting cool it clots all the same, in fact 

 quicker, and if kept cold enough will not clot at all. 

 Nor does it clot when shed, because it has become 

 st;ll, and is no longer rushing round through the 

 blood-vessels. Nor is it because it is exposed to 

 the air. Perhaps we don't know exactly why it is, 

 and you will have much to learn hereafter about 

 the coagulation of blood. All I will say at present 

 is that as long as the blood is in the body there 

 is something at work to keep it from clotting. It 

 does clot sometimes in the body, and blood-vessels 

 get plugged with the clots; but that constitutes a 

 very dangerous disease. 



24. Well, blood is thicker than water because it 

 contains solid corpuscles and fibrin. But even the 

 serum, ue, blood out of which both fibrin and cor- 

 puscles have been taken, is thicker than water. 



You know that if you were to take a basinful of 

 pure water and boil it, it would boil away to nothing. 

 i| It would all go off in steam. But if you were to try 

 f to boil a basinfiil of serum, you would find several 

 curious things happen. 



In the first place you would not be able to boil it 

 at all. Before you got it as hot as boiling water, your 

 I serum, which before seemed quite as liquid as water, 

 only feeling a little sticky if you put your finger in it, 

 I would all become quite solid. You know the differ- 

 ence between a raw and a boiled egg. The white of 

 the raw egg, though very sticky and ropy, or viscid as 



