ACID 17 



rod precipitate when brought in contact with an ammoniacal solution of subchloride 

 of copper. This red compound explodes either by percussion, or on being suddenly 

 heate^l to a temperature a little above that of Roiling water. It has been suggested 

 that this is probably the cause of certain gas explosions which have occurred in un- 

 screwing the brass fittings of gas-meters. Acetylene is one of the constituents of 

 coal gas, and the red explosive compound is liable to be formed by contact of the gas 

 with the brass-work. 



ACHROMATIC, destitute of colour. White light consists, as is shown by its de- 

 composition by a prism, of several coloured rays, having different degrees of refrangi- 

 bility. When, therefore, white light passes through any transparent body, such as 

 a lens, it is liable to this decomposition to a greater or a less extent, and hence 

 colour is produced. This is termed chromatic aberration. Many, especially old- 

 fashioned, telescopes exhibit objects surrounded by beautifully coloured fringes. 

 Now the means which have been devised to prevent this are termed achromatic, 

 signifying the deprivation of colour. See LIGHT. 



ACHROMATIC IiENS. Hale, in 1733, constructed lenses which did not pro- 

 duce chromatic dispersion. In 1757 Dollond arrived, by a perfectly independent 

 examination, at the same discovery, and published it. 



A lens may be regarded as a number of prisms united round a centre ; therefore a 

 ray of light falling on a lenticular glass is decomposed, and the rays being of unequal 

 refrangibility, they have on its axis as many foci as there are colours. The images, 

 therefore, of objects which are produced at these points are superimposed, more or 

 less, and the edges fringed with indistinct colours. The least refrangible rays unite 

 at foci further away than the more refrangible ; and the object sought for, and 

 attained, by both Hale and Dollond, was the means of uniting these rays at one focal 

 point. They combined flint-glass with crown-glass, and found that, by a suitable 

 curvature given to the object-glasses, the images seen through them were distinct, and 

 free from these adventitious colours. 



Telescopes, microscopes, &c., fitted up with such combinations of lenses as those 

 described, are called achromatic telescopes. 



AClCTTlilTE. A name applied to Aikenite (a native sulphide of bismuth, 

 copper and lead), in allusion to its occurrence in acicular, or needle-like crystals. See 



ArKEOTTE. 



ACID. (Acidus, sour, L.) The term acid was formerly applied to bodies which 

 were sour to the taste, and in popular language the word is still so used. It is to be 

 regretted that the necessities of science have led to the extension of this word to any 

 bodies combining with bases to form salts, whether such combining body is sour or 

 otherwise. Had not the term acid been established in language as expressing a sour 

 body, there would have been no objection to its use ; but chemists now apply the term 

 to substances which are not sour, and which do not change blue vegetable colours ; 

 and consequently they fail to convey a correct idea to the popular mind. 



Hobbes, in his ' Computation or Logic,' says, ' A name is a word taken at pleasure 

 to serve for a mark which may raise in our mind a thought like to some thought we 

 had before, and which, being pronounced to others, may be to them a sign of what 

 thought the speaker had, or had not, before in his mind.' This philosopher thus truly 

 expresses the purpose of a name ; and this purpose is not fulfilled by the term acid, 

 as now employed. 



Mr. John Stuart Mill, in his ' System of Logic,' thus, as it appears not very hap- 

 pily, endeavours to show that the word acid, as a scientific term, is not inappropriate 

 or incorrect. 



' Scientific definitions, whether they are definitions of scientific terms, or of common 

 terms used in a scientific sense, are almost always of the kind last spoken of : their 

 main purpose is to serve as the landmarks of scientific classification. And, since the 

 classifications in any science are continually modified as scientific knowledge advances, 

 the definitions in the sciences are also constantly varying. A striking instance is 

 afforded by the words acid and alkali, especially the former. As experimental dis-' 

 covery advanced, the substances classed with acids have been constantly multiplying ; 

 and, by a natural consequence, the attributes connoted by the word have receded and 

 become fewer. At first it connoted the attributes of combining with an alkali to form 

 a neutral substance (called a salt), being compounded of a base and oxygen, causticity 

 to the taste and touch, fluidity, &c. The true analysis of muriatic acid into chlorine 

 and hydrogen caused the second property, composition from a base and oxygen, to be 

 excluded from the connotation. The same discovery fixed the attention of chemists 

 upon hydrogen as an important element in acids ; and more recent discoveries having 

 led to the recognition of its presence m sulphuric, nitric, and many other acids, where 

 its existence was not previously suspected, there is now a tendency to include the 

 presence of this element in the connotation of the word, But carbonic acid, silica, 



VOL. I. C 



