THE CHEMISTRY OF CLEANING. 103 



sink into the fabric, nor so clinging as to resist easy removal by 

 the broom. 



It is manifest, however, that the mechanically held dirt which 

 we have been considering differs very considerably from the dirt 

 on our skins, and on linen in contact with oiir bodies, which, al- 

 though derived from the same sources as the dust on the furniture, 

 resists any ordinary mechanical process for its removal, and rins- 

 ing dirty hands or linen in cold water has but little cleaning 

 effect, while if the hands are afterward dried in the usual way a 

 transfer of a portion of dirt to the towel takes place. If we care- 

 fully notice the portions of our skin and shirt which become most 

 soiled, we at once observe that it is where the skin is exposed to 

 air, while the linen, which is in contact with both air and skin, 

 becomes dirty more quickly than when exposed to either alone. 



The part played by the atmosphere is made clear by the facts 

 which we have already been considering, but the action of the 

 skin introduces a new and most important factor. For the 

 healthy carrying on of the functions of life, nothing exceeds in 

 importance the skin with which our body is covered. We may 

 live for days without giving our stomach any work to do, the 

 liver may cease action for several days before death ensues, but 

 it is impossible to survive for the same length of time if the func- 

 tions of the skin are entirely stopped. The skin not only plays an 

 important part in throwing off and getting rid of waste matter 

 from the system, but it is also credited with being an important 

 auxiliary to our lungs ; and experiments have clearly shown that 

 if the skin of animals be coated in such a way as to completely 

 stop its action, a very few hours will bring about death. 



If we examine the structure of the skin, we find that it is built 

 up of two distinct layers, an outer skin called the cuticle or epi- 

 dermis, and an inner termed the cutis or dermis. A third layer 

 intermediate between these two used to be looked upon as a third 

 skin, but more recently has been recognized as being only a tran- 

 sition form- of the outer skin. The cuticle or outer skin consists 

 of several fine layers of scales which gradually assume a more 

 rounded and granular form the deeper one gets into the cuticle. 

 These rounded granules form the middle skin of the old observers, 

 and as the outer portion of the cuticle roughens and scales off as 

 scurf, these granules gradually flatten and form the new surface 

 to the outer skin ; and we differ therefore from other scaly reptiles 

 by being continually in a condition of renewing our skin, while 

 most reptiles and fish cast their scaly covering in one operation. 



No nerves or blood-vessels find their way into this outer skin, 

 as may be seen when it becomes detached from the inner skin in 

 the formation of a blister, the outer portion of which is devoid 

 of sensation. The lower or true skin varies in thickness, being 



