3!0 SCIENTIFIC RECREATIONS, 



standard atom is hydrogen, which is put down as i, because we find that 

 when one part by weight of hydrogen is put in combination, it must have 

 many more parts by weight of others to form a compound. Two grains 

 of hydrogen, combining with sixteen of oxygen, makes eighteen of water, as> 

 we have already seen. 



Take an example so plainly given by Professor Roscoe, remembering 

 that the numbers in our table represent the fixed weight or proportion by 

 weight in which the simple body combines. The red oxide of mercury 

 contains sixteen parts by weight of oxygen to two hundred parts by weight 

 of mercury (we see the same numbers in the table) ; these combined make 

 two hundred and sixteen parts of oxide. So to obtain 16 Ibs. of oxygem 

 we must get 216 Ibs. of the powder. It is the same all through, and it wilt 

 be found by experiment that if any more parts than these fixed proportions 

 be taken to form a compound, some of that element used in excess will 

 remain free. Lime is made up of calcium and oxygen. We find calcium 

 combining weight is forty, oxygen sixteen. Lime is oxide of calcium in 

 these proportions (by weight). 



When we wish to express the number of atoms in a compound we write 

 the number underneath when more than one; thus water is H 2 O. Sulphuric 

 acid H 2 SO 4 . As we proceed we will give the various formula when con- 

 sidering the chief elements. 



In chemistry we have acids, alkalis, and salts, with metallic oxides, 

 termed doses, or bodies, that when combined with acids form salts. Alkalis 

 are bases. 



ACIDS are compounds which possess an acid taste, impart red colour to 

 vegetable blues, but lose their qualities when combined with bases. Hydrogen 

 is present in all acids. There are insoluble acids. Silicic acid, for instance > 

 is not soluble in water, has no sour taste, and will not redden the test litmus 

 paper. On the other hand, there are substances (not acids) which possess 

 the characteristics of acids, and most acids have only one or two of these 

 characteristics. 



Thus it has come to pass that the term " acid " has in a measure dropped 

 out from scientific nomenclature, and salt of hydrogen has been substituted 

 by chemists. For popular exposition, however, the term is retained. 



ALKALIS are bases distinguished by an alkaline taste. The derivation: 

 is from Arabic, al-kali. They are characterized by certain properties, and they 

 change vegetable blues to green, and will restore the blue to a substance 

 which has been reddened by acid. They are soluble in water, and the 

 solutions are caustic in their effects. Potash, soda, and ammonia are alkalis, or 

 chemically, the oxides of potassium ; sodium, ammonium, lithium, and caesium 

 are all alkalis. Potash is sometimes called " caustic" potash. There are alka- 

 line earths, such as oxides of barium, strontium, etc. Bases may be defined as 

 the converse of acids. 



Acids and alkalis are then evidently opposite in character, and yet they 

 readily combine, and in chemistry we shall find that unlike bodies are very 



