AGENCY OF WATER. 45 



hardness is unknown. A greater or smaller tenacity is the 

 characteristic which best marks out the textures pre-eminently 

 meriting the name of solids. Nevertheless, it is common to 

 attach much importance to the division of the component parts 

 of the animal body into solids and fluids. When such a solid 

 as bone is contrasted with such a fluid as saliva, the distinction 

 seems broad and palpable. But when brain, which ranks as 

 an animal solid, is put side by side with blood, which ranks as 

 an animal fluid, the distinction appears to be less natural than 

 conventional. Brain has few of the characters of a solid ; it 

 has little tenacity, and but a small power that which is char- 

 acteristic of a solid of maintaining figure against the influence 

 of gravity. The blood, on the other hand, though a fluid in 

 the hydraulic sense, as capable of motion under ordinary im- 

 pulses, is very far from being a fluid in the chemical sense, 

 owing to the large proportion of undissolved solid matter 

 which it contains. The difference in density between the two 

 bodies shows the advantage on the side of the blood brain 

 having an average density of 1.036, and blood that of 1.052. 

 It has to be remarked, however, that what is here said is true 

 in particular of the grey cerebral matter, as the white cerebral 

 matter owes its similar deficiency in the true characters of a 

 solid not so much to the large proportion of water as to the 

 great amount of fluid fat contained in its structure. The urine 

 is sometimes as dense as healthy blood ; yet it does not there- 

 fore lose the character of a true chemical fluid, inasmuch as 

 the naturally solid substances which it contains are completely 

 dissolved in the water present. 



But to return to the uses of this large proportion of water 

 spread everywhere throughout the animal economy : it is mani- 

 fest that the organic atoms which compose the solids of the 

 animal body exist in a fluid medium very much of the same 

 kind, though less in degree, as that in which the organic atoms 



