53 



ACETYLAMINE. 



ACIDS. 



(54 



radical of acetic compounds, acetic aldehyde being, according to his 

 view, hydrated oxide of acetyl (C 4 H 3 O, HO), whilst acetic acid was 

 regarded as the hydrated teroxide of acetyl (C 4 H,0 3 HO). Secondly, 

 the name has more recently been applied by Gerhardt to the group 

 (C 4 H,Oj) which is regarded by the latter chemist as the true radical of 

 iicetic compounds, acetic aldehyde being the hydride of this radical 

 (C,H 3 0,, H), and acetic acid its hydrated oxide (C.H.,0,0, HO). Wil- 

 liainson proposes the name othyl for the group (CjHjOJ. [NEGATIVE 

 RADICALS.] 



ACETYLAMINE (Acetylia). An organic base first obtained by 

 M. Cloez, and, subsequently, by M. Natanson, in acting upon bibromide 

 of ethylene and bichloride of ethylene with ammonia, and regarded by 

 these chemists as having the formula C 4 H 5 N. The recent researches 

 of Hofmann prove this body to belong to the family of diamines ; 

 according to this chemist it is (iietlo/lenc-diamine, and its formula is 

 " 



...,,. iAMlXES. 



ACHROMATIC (from a idthouf, and XP a M a flour), a term applied 

 to those combinations of lenses xtsed in the best telescopes and micro- 

 Hcopes, for preventing the formation of coloured fringes which surround 

 the edges of objects when viewed by means of common instruments. 

 [LIGHT.] 



ACIDIMETRY, the process of determining the quantity of real 

 acid contained in a given sample of any acid, and thereby ascertaining 

 its actual or intrinsic value. 



There are various methods of accomplishing this : the simplest is by 

 determining the specific gravity of the acid in question. As in most 

 cases the specific gravity of an acid diminishes in regular proportion to 

 the amount of water it contains ; the amount of real acid is easily 

 calculated from its density. To facilitate this, tables have been con- 

 structed by Dr. Ure and others, in which the specific gravity and the 

 amount of real acid corresponding to it, are placed in parallel columns. 

 These tables will be given in describing the various acids. 



The above method however is not always absolutely accurate, and 

 some acids do not admit of its use at all. Advantage has therefore 

 been taken of the fact, that the blue colour of litmus is reddened 

 l*y acids, while alkalies restore the original colour, to construct a 

 method of estimation which is highly accurate and expeditious. An 

 alkaline solution is prepared of a known strength, and is poured 

 from a graduated tube an alkalimeter into an accurately weighed 

 quantity of the acid to be examined (which must be tinged red with 

 litmus) till the point of neutralisation, known by the change from red 

 to blue, is reached. From the number of measures of the test liquid 

 .*> used it is easy to calculate the quantity of real acid in the sample 

 tested. 



A convenient method of preparing the test liquid is to dissolve 

 330 grains (10 equivalents) of pure dry carbonate of soda made by 

 fusing the purr bicarbonate in 10.000 grains of distilled water. The 

 nlkaumeter should be made to hold 1000 gram measures, and would 

 ' ontain 53 grains, or an equivalent of carbonate of soda. It 

 should be divided into 100 irt*. each of which will then contain 

 '53 of the carbonate. An equivalent of carbonate of soda will exactly 

 neutralise an equivalent of acid, so that 100 grain measures or divisions 

 of the test liquid will represent 



40 grains of sulphuric acid (SO,), or 



49 Krain.t of oil of vitriol (HO, SO,), 



36-5 grains of dry hydrochloric acid (IIC1), 



o4 grains of nitric acid (NOJ, or 



61 gnliu (HO, NO,), 



60 grains of hydrated (glacial) acetic acid (IK>,C 1 H.,n ,), &c., &c. 



.Supposing, then, that 100 grains of the sample of acid to be tested 

 are weighed, and they require 70 measures of the test liquid, if the 

 ." il be sulphuric, then 100 : 40 : : 70 : 28, the amount of real acid in 

 th quantity taken; if hydrochloric, 100 : 36'5 : : 70 i 25'55,and so on. 

 A solution of ammonia may be substituted for that of carbonate of 

 soda, and may be so adjusted that 1000 grain measures shall contain 

 17 grains of ammonia, which is the case when its specific gravity 

 reaches '992. It is not easy however to adjust it exactly to such a 

 density ; it is better, therefore, when near the convenient strength to 

 estimate the ammonia in a given number of measures of the alkalimeter, 

 by ev;ij>orating them to dryness on a water-bath, with an excess of 

 biiihlnride of platinum. The resulting platinum salt, after being 

 wanlwjcl on a filter, with a mixture of 2 parts of alcohol with 1 of 

 and carefully dried at 212", and weighed, contains in 100 part*, 

 iini.nia 



Tin: strength of flic test ammonia may in this way be made to suit 



t the operator, and the amount of ammonia in each 



livision being known, the calculation is perfectly simple. Suppo..in^ 



that each measure contains '20 grains of ammonia, and that 100 weighed 



grains of the sample of sulphuric acid require 50 measures, the eq. of 



nia, 17 : 40 : : 50 x -20 : 23'53 grains of sulphuric acid in the 



quantity taken. 



are other processes for accomplishing the same object. Those 



iy, which will be fully described, may, in many cases, 



be used in a reverse way in Acidiiuetry, the one process being the 



reverse of the other. Home acids require special methods of analysis, 



which will be described under their particular titles. 



ACIDS. The acids are a numerous and important class of chemical 

 bodies. As the word acid is, in common language, almost synonymous 

 with sour, it might be supposed that the taste of a substance would 

 determine whether it was included among the acids. The term has. 

 however, been so much extended by chemists beyond its original 

 meaning, that some bodies, which are nearly or quite devoid of sour- 

 ness, are considered as acids because they agree with them in some 

 other qualities. The acids are generally sour ; usually, but not 

 universally, they have great affinity for water, and are readily soluble in 

 it : they redden most vegetable blue colours, and combine readily 

 with alkalies and earths, and generally act upon and unite with most 

 metals or then- oxides, with great facility, forming compounds which are 

 termed salts. Such are the properties of the greater number of acids ; but 

 the last only, namely the power of combining with bases, belongs to 

 them all. Many acids are entirely natural products, some both natural 

 and artificial, while others are altogether the result of chemical agency. 

 They are derived from various sources, and, except in the few par- 

 ticulars above-named, vary greatly in their properties. Thus, under 

 common circumstances of temperature and pressure, some are gaseous 

 in form, as the carbonic acid ; others are fluid as the nitrous, or solid, 

 as the boracic acid ; some require water or a base to retain their 

 elements in combination, which is the case with the oxalic acid, 

 while others, as the sulphuric and nitric may exist independently 

 of either. Most acids are colourless, but the chromic is red ; some 

 are inodorous, as the sulphuric ; others pungent, as the hydrochloric 

 acid ; there are acids which are comparatively fixed in the fire, the 

 phosphoric for example ; others are volatilised by a more moderate 

 heat, which is the case with the sulphuric acid ; whilst those which 

 are pungent to the smell are, to a certain extent, volatile at all 

 temperatures. 



Acids occur in all the kingdoms of nature : the margaric acid is of 

 animal origin ; the citric and the oxalic acid are products of vegetation ; 

 while the chromic and the arsenic acid enter into the composition of 

 certain minerals. In many instances however acids are not exclusively 

 derived from one source, but are sometimes produced by them all, and 

 may be also artificially formed. This is the case with the phosphoric 

 acid, which occurs in animals, plants, and minerals, and is formed 

 whenever phosphorus is burnt in excess of oxygen. The citric acid 

 is produced only by the process of vegetation ; but the oxalic acid, 

 also found in plants, may be obtained by chemical agency. The 

 carbonic and the sulphuric acid are very Common in mineral bodies, 

 and may also be artificially produced ; the former is also one of 

 the results of respiration, combustion, and of animal and vegetable 

 decomposition ; and both the carbonic and sulphuric acids may be 

 obtained by combining carbon and sulphur respectively with oxygen. 

 The chromic and the arsenic acid are found only in mineral bodies, but 

 they may be formed by chemical agency ; and indeed, except many 

 of the vegetable acids, there are but few which cannot be so prepared. 



Soon after Dr. Priestley's celebrated and important discovery of what. 

 he called dephloyitticated air, in 1774, it was found that several sub 

 stances, such as sulphur and phosphorus, were converted into acids by 

 combining with this elementary gas. On this account it was assumed, 

 hastily and incorrectly, that all acids contained dephlogisticated air, 

 and derived their acidity from it ; on this account the name oxygen 

 was given to it, signifying arid-making, and it was regarded as the 

 universal acidifying principle ; not indeed that it always formed an acid 

 when combined with a body, but that no acid existed without it. It 

 has however since been found that there are acids, the hydrochloric 

 acid for example, which contain no oxygen ; and further, it has also 

 been proved, by the brilliant discoveries of Sir H. Davy, that oxygen, 

 by combining with certain elementary bodies, converts them into 

 ,ill,-nlii:i ; a class of substances possessing properties diametrically 

 opposite to those of the acids. 



It was therefore considered necessary to divide the acids into oxyacicln 

 in which oxygen was supposed to form the acidifying principle ; and 

 hydraridt in which that principle was due to hydrogen. Hydrated 

 sulphuric acid, HO, SO, may be considered as a type of the former 

 class ; hydrochloric acid, HCl, of the latter class of acids. When 

 sulphuric or any oxyacid is united to a metallic oxide, the result is a 

 salt in which the water of the acid is replaced by the oxide, forming 

 a so-called o.fy*alt. Thus HO,S0 3 + NaO = NaO,S0 3 + HO. When 

 hydrochloric acid or any hydracid is so combined, the hydrogen is 

 replaced, not by the oxide, but by the metal itself. Thus, HCl + NaO 

 = NaC'l + HO, or chloride of sodium, a salt containing neither oxygen nor 

 hydrogen, and called a haloid salt (from &\s, the sea), sea-salt being the 

 type of such a compound. Thus the two classes of acids produce in 

 their combinations apparently anomalous results. T obviate this, 

 it was suggested by Sir H. Davy, and has since been supported by 

 (Jraham, Liebig, and others, that all acids are hydracids, and all salts 

 haloid salts. By this theory, an oxyacid is in all cases a combination of 

 hydrogen with a compound salt-radical. Thus sulphuric acid, instead 

 of being HO, SO, is H+ the salt radical SO, , or HS0 4 . Nitric acid, 

 not HO,NO,, but H,N0 , and so on. In the formation of salts, there- 

 fore, the hydrogen of the acid is simply replaced by a metal, as in 

 common salt 



NaO + HCl = Nad + HO. 

 NaO + HSO. = NaS0 4 + HO. 

 NaO 4- HNO n = NaNO. + HO. 



