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The comparative zocight of each atom being;thus ascertained, the relative diameter of theatom is found by comparing its relative weightwith the specific gravity of the substance ofwhich the atom is an integrant particle. Butas this part of Mr. Dalton’s system is not es-sential to our present purpose we shall notpursue it.

Not only do elementary atoms unite in defi-nite proportions but also compound particlesunite in the same manner. Thus both sul-phuric acid and potash are compound particles,being each oxyds, the one of sulphur and theother of potassium. But as potash combineswith two different proportions of sulphuricacid, one being just double the quantity of theother, the compound, sulphat of potash, maywith propriety be considered as composed of anatom of potash with an atom of sulphuric acid,and the compound, super-sulphat of potash ,may be considered as consisting of one atom ofpotash to two atoms of sulphuric acid.

. Mr. Dalton gives the term Binary atom toany compound of two elements in which oneatom of each is combined, and hence therecan be but one species of binary compound ofany two elements. t

A Ternary Atom is composed of two atomsof one of the elements with one atom of theOther, and hence there may be two species ofternary atoms of the same element, acco; ding asone or other is in the greater proportion. Thusan atom of nitrous oxyd is a ternary compoundof two atoms of azote and one atom of oxygen;and on the other hand nitrous acid is also aternary compound of the same elements, butconsisting of one atom of azote with two atomsof oxygen.

A Quaternary Atom is composed of threeatoms of one element with one atom of theother, and hence also there may be two speciesof quaternary atoms as either element predomi-nates ; and so on of the other numbers.

We shall now proceed to mention a numberof facts that illustrate in a striking manner thechemical union of substances in definite pro-portions.

If one measure of pure oxygen and twomeasures of hydrogen be mixed in a jar overmercury and ignited by the electric spark, boththe gasses will disappear, and water will be pro-duced. If two measures of each gas be used,water will be produced as before, but one mea-sure of oxygen will remain. Hydrogen there-

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fore unites with water in one exact proportion,and in no other.

If a piece of well burnt charcoal be confinedin oxygen gas and inflamed by a burning glassthe volume of gas is not altered when againcooled, but the whole is converted into carbonicacid gas. If more oxygen be present than isnecessary for the consumption of the charcoal,the products will be carbonic acid gas and anexcess of oxygen; if there is less oxygen thanwill consume the charcoal, carbonic acid alonewill be produced, and part of the charcoal willremain unconsumed.

The combination of two elements in severaldefinite proportions is very happily shewn bythe various compounds of oxygen and azote.These are nitrous oxyd, nitrous gas, and nitrousacid gas.

If two measures of Nitrous Oxyd , and twomeasures of hydrogen are ignited by the electricspark, the product is water, and two measuresof azote remain. Now as water is producedby two measures of hydrogen and one of oxygenthe nitrous oxyd here employed must have-consisted of two measures of azote with one ofoxygen condensed into the space of one measure.

If charcoal is ignited in two measures ofNitrous gas the products are one measure ofcarbonic acid gas and one measure of azote.Hence, as carbonic acid gas always occupiesthe same volume as the oxygen of which it isformed, nitrous gas consists of equal volumesof oxygen and of azote not condensed by theirunion.

If two measures of nitrous gas be mixedover water with one measure of oxygen gas,both of them totally disappear, and a solutionof Nitrous acid gas in water is the result.

In all the above examples the proportions ofazote and oxygen encrease or diminish by equalquantities, and no intermediate states of combi-nation are known.

Dr. Wollaston has given some striking expe-riments in illustration of this theory of definiteproportions in the composition of some super-acid and sub-acid salts, 6 which may be here-mentioned as they are easily performed. Let twograins of crystallized carbonat of potash re-cently prepared be wrapped in thin paper, andpassed up into an inverted tube filled with mer-cury, and let the gas be extricated from it bymuriatic acid, and the space it occupies bemarked on the tube. Then let four grains ofthe same carbonat be exposed for a short time to>

Phil. Trans, for 1808 ..