328 CHEMICAL DISCOVERY AND INVENTION 



has yet been completely established, and much remains in 

 obscurity. 



It would perhaps be advisable at this point to remind the non- 

 chemical reader of the meaning attached to the words acid and 

 base. 



Acids are substances such as acetic acid in vinegar, tartaric, 

 citric, and malic acids found in fruit, and the mineral acids, 

 sulphuric, nitric, and hydrochloric acids. These are all soluble 

 in water, and when sufficiently diluted have a sour taste and 

 cause chalk and other carbonates to effervesce when mixed with 

 them. 



Basic substances are those which when mixed with an acid in 

 due proportion destroy the acid taste and give rise to a new 

 compound called a salt. Basic substances are divisible into two 

 classes, of which one is represented by ammonia, aniline, and other 

 organic bases. These combine directly with acids. The other 

 class of basic compounds includes hydrated oxides of metals, 

 such as caustic soda, lime, alumina, etc. When these are 

 mixed with an acid they produce a salt,' and the oxygen 

 of the base unites with the hydrogen of the acid forming 

 water. 



The word salt is used in a very wide sense by the chemist, and 

 though common salt is the most familiar of all salts, they are 

 not all, like it, soluble in water and neutral, that is neither sour 

 nor soapy to the taste. 



Wool and silk are known to be composed chiefly of substances 

 which are very complex in constitution, but when decomposed 

 they yield peculiar compounds which behave under one set of 

 conditions as acids, and under another as bases. The simplest 

 example of such compounds is glycocoll (now commonly called 

 glycine) or amino-acetic acid, NH 2 -CH 2 -C0 2 H, which will 

 produce a salt either by combining with a base or, in virtue of the 

 ammonia residue, NH 2 , which it contains, by combining with 

 an acid such as hydrochloric acid. 



This peculiar property is shared by the substances of which 

 silk and wool consist, and they are therefore able to enter into 

 chemical combination with dyeing matters of both an acid and 

 basic character. 



When, for example, silk or wool is dyed with magenta, which 

 is a salt of a basic dye, the colour base leaves the mineral acid 

 with which it is combined and unites with the acidic constituent 



