76 Domestic Science 



intervals of about two hours during the daytime. It 

 will be found that the lower portion becomes reddened 

 by the acid and that the action of the acid gradually 

 extends upwards till the whole of the liquid is red, in 

 spite of the considerable difference in density of -the 

 acid and the litmus solution. 



These experiments illustrate the diffusive power of 

 fluid bodies, a power only possessed by solids to an 

 extremely limited extent. That solids do diffuse, at 

 a rate which is very small indeed compared with that 

 at which liquids and gases intermingle, seems to be 

 shown by such phenomena as the absorption of charcoal 

 (carbon) by hot iron when the carbon is closely packed 

 around it, with the consequent formation of steel, and 

 the finding of appreciable traces of gold in the upper 

 layers of a leaden bar to the top of which a piece of 

 gold has been applied under pressure for some years. 



45. When scientists discover such interesting facts 

 as those we have illustrated in the experiments on 

 diffusion, they try to form an idea of the probable 

 cause of the observed phenomena. ("Phenomenon" 

 simply means an " appearance ", i.e. something seen 

 or observed.) This generally involves the making of 

 some supposition or assumption with regard to the 

 nature of the bodies under examination and such an 

 assumption is termed a "hypothesis". Hypotheses 

 of this character are usually incapable of direct experi- 

 mental proof, but are frequently of the greatest service 

 in helping us to form a mind-picture of what may be 

 going on during an experiment, and are further of 

 value in that their consideration may suggest that 

 other properties, hitherto unsuspected, are possessed 

 by the substances dealt with. The existence or 

 otherwise of such properties may be tested by further 



