304 ERGOT OF RYE 



2. Mode of determining the equivalent of an alkali. Several methods present them- 

 selves, each possessing certain advantages. Most alkalis, organic and inorganic, 

 form salts well adapted for enabling their atomic weight to bo ascertained by analysis. 

 Wo shall select as an example ammonia, and the salt employed to settle the atomic 

 weight will be the sulphate, which contains 



Oxide of ammonium 39*40 



Sulphuric acid 60-60 



100-00 



In the same way that an oxide of known composition is the datum employed to 

 determine the equivalent of an acid, so, on the other hand, an acid, the formula of 

 which is well established, servos to enable the formula of an alkali to bo deduced. Wo 

 therefore say: 



60-60 39-40 40-00 26'00 



Percentage of acid. Percentage of alkali. Equivalent of the Equivalent of the 



acid. alkali. 



Most alkalis, especially those derived from the organic kingdom, form well-defined 

 and easily cry stalli sable compounds with some of the metallic chlorides, especially 

 those of gold, platinum, and palladium. These salts are well adapted for enabling 

 atomic weights to be fixed. 



3. Mode of determining the equivalent of a neutral substance. Neutral bodies are 

 formed upon so many models or types that no general method can bo given for the 

 required purpose. If volatile at moderate temperatures, the density of the vapour am 

 be ascertained, and this is generally sufficient. Salts have their equivalents found 

 by determining the percentage composition, and proceeding as in Examples 1 and 2. 

 The equivalent of a metal is found by forming a compound with some substance, the 

 atomic weight of which is well known, such as oxygen or sulphur. The compound is 

 then carefully analysed. EXAMPLE: It has been found that 100 parts of oxide of 

 copper contain 



Copper SO'OO 



Oxygen 20'00 



100-00 

 We therefore say : 



20-00 so-oo :: s-oo 32-00 



Percentage of Percentage of Equivalent of 



oxygen. copper. oxygen. 



A precisely analogous mode of proceeding may be adopted with chlorides, iodides, 

 &c. C.G.W. 



It is important to clearly separate the idea of chemical equivalence from that of 

 atomic weight. Two elements can be strictly called equivalent to each other only when 

 one is capable of being substituted for the other in chemical combination. Thus 

 35'5 parts of chlorine (its atomic weight) are strictly equivalent to 1 part of hydro- 

 gen; but 16 parts of oxygen (its modern atomic weight) are equivalent to 2 j-.irts 

 of hydrogen; 14 parts of nitrogen usually to 3 of hydrogen; 12 of carbon to 4 of 

 hydrogen ; and so on. Hence the idea of chemical equivalence is by no means tlio 

 same as that of atomic weight ; the atom of one element not being necessarily cijui va- 

 lent to another in its combining power. For the modern doctrine of equivalence, 

 see Watts's ' Dictionary of Chemistry,' and in this work, the articles ATOMICITY 

 and ATOMIC WEIGHTS. 



ERASINE. See AUIKTEXK. 



ERBIUM. The metallic radicle of Erbia, one of the earths associated with yttria. 

 It was discovered byMossandor in 1842. Salts of orbia give characteristic absorption- 

 spectra exhibiting dark bands. See Watts's ' Dictionary of Chemistry.' 



EREM ACAUSIS, slow combustion. This term has been applied to that constant 

 combination of oxygen with carbon and hydrogen, to form carbonic acid and water, 

 which is unceasingly going on in nature, as in the decay of timber or the ' heating ' 

 of hay or grain put together in a moist state. Perfect dryncss, and a temperature 

 below freezing, stop this eremacausis, or slow combustion. 



ERGOT OP RYE. Diseased grains of rye ('spurred rye ') which become 

 elongated and curved in shape, and of a jet-black colour, by the attack of a fungus 



