〃We have； therefore； laid aside the expression metallic calx altogether； and have substituted in its place the word oxide。 By this it may be seen that the language we have adopted is both copious and expressive。 The first or lowest degree of oxygenation in bodies converts them into oxides； a second degree of additional oxygenation constitutes the class of acids of which the specific names drawn from their particular bases terminate in ous； as in the nitrous and the sulphurous acids。 The third degree of oxygenation changes these into the species of acids distinguished by the termination in ic； as the nitric and sulphuric acids； and； lastly； we can express a fourth or higher degree of oxygenation by adding the word oxygenated to the name of the acid； as has already been done with oxygenated muriatic acid。〃＇9＇
This new work when given to the world was not merely an epoch…making book； it was revolutionary。 It not only discarded phlogiston altogether； but set forth that metals are simple elements； not compounds of 〃earth〃 and 〃phlogiston。〃 It upheld Cavendish's demonstration that water itself； like air； is a compound of oxygen with another element。 In short； it was scientific chemistry； in the modern acceptance of the term。
Lavoisier's observations on combustion are at once important and interesting： 〃Combustion；〃 he says； 〃。 。 。 is the decomposition of oxygen produced by a combustible body。 The oxygen which forms the base of this gas is absorbed by and enters into combination with the burning body； while the caloric and light are set free。 Every combustion necessarily supposes oxygenation； whereas； on the contrary； every oxygenation does not necessarily imply concomitant combustion； because combustion properly so called cannot take place without disengagement of caloric and light。 Before combustion can take place； it is necessary that the base of oxygen gas should have greater affinity to the combustible body than it has to caloric； and this elective attraction； to use Bergman's expression； can only take place at a certain degree of temperature which is different for each combustible substance； hence the necessity of giving the first motion or beginning to every combustion by the approach of a heated body。 This necessity of heating any body we mean to burn depends upon certain considerations which have not hitherto been attended to by any natural philosopher； for which reason I shall enlarge a little upon the subject in this place：
〃Nature is at present in a state of equilibrium； which cannot have been attained until all the spontaneous combustions or oxygenations possible in an ordinary degree of temperature had taken place。。。。 To illustrate this abstract view of the matter by example： Let us suppose the usual temperature of the earth a little changed； and it is raised only to the degree of boiling water； it is evident that in this case phosphorus； which is combustible in a considerably lower degree of temperature； would no longer exist in nature in its pure and simple state； but would always be procured in its acid or oxygenated state； and its radical would become one of the substances unknown to chemistry。 By gradually increasing the temperature of the earth； the same circumstance would successively happen to all the bodies capable of combustion； and； at the last； every possible combustion having taken place； there would no longer exist any combustible body whatever； and every substance susceptible of the operation would be oxygenated and consequently incombustible。
〃There cannot； therefore； exist； as far as relates to us； any combustible body but such as are non…combustible at the ordinary temperature of the earth； or； what is the same thing in other words； that it is essential to the nature of every combustible body not to possess the property of combustion unless heated； or raised to a degree of temperature at which its combustion naturally takes place。 When this degree is once produced； combustion commences； and the caloric which is disengaged by the decomposition of the oxygen gas keeps up the temperature which is necessary for continuing combustion。 When this is not the casethat is； when the disengaged caloric is not sufficient for keeping up the necessary temperaturethe combustion ceases。 This circumstance is expressed in the common language by saying that a body burns ill or with difficulty。〃＇10＇
It needed the genius of such a man as Lavoisier to complete the refutation of the false but firmly grounded phlogiston theory； and against such a book as his Elements of Chemistry the feeble weapons of the supporters of the phlogiston theory were hurled in vain。
But while chemists； as a class； had become converts to the new chemistry before the end of the century； one man； Dr。 Priestley； whose work had done so much to found it； remained unconverted。 In this； as in all his life…work； he showed himself to be a most remarkable man。 Davy said of him； a generation later； that no other person ever discovered so many new and curious substances as he； yet to the last he was only an amateur in science； his profession； as we know； being the ministry。 There is hardly another case in history of a man not a specialist in science accomplishing so much in original research as did this chemist； physiologist； electrician； the mathematician； logician； and moralist； the theologian； mental philosopher； and political economist。 He took all knowledge for his field； but how he found time for his numberless researches and multifarious writings； along with his every…day duties； must ever remain a mystery to ordinary mortals。
That this marvellously receptive； flexible mind should have refused acceptance to the clearly logical doctrines of the new chemistry seems equally inexplicable。 But so it was。 To the very last； after all his friends had capitulated； Priestley kept up the fight。 From America he sent out his last defy to the enemy； in 1800； in a brochure entitled 〃The Doctrine of Phlogiston Upheld；〃 etc。 In the mind of its author it was little less than a paean of victory； but all the world beside knew that it was the swan…song of the doctrine of phlogiston。 Despite the defiance of this single warrior the battle was really lost and won； and as the century closed 〃antiphlogistic〃 chemistry had practical possession of the field。
III。 CHEMISTRY SINCE THE TIME OF DALTON
JOHN DALTON AND THE ATOMIC THEORY
Small beginnings as have great endingssometimes。 As a case in point； note what came of the small； original effort of a self…trained back…country Quaker youth named John Dalton； who along towards the close of the eighteenth century became interested in the weather； and was led to construct and use a crude water…gauge to test the amount of the rainfall。 The simple experiments thus inaugurated led to no fewer than two hundred thousand recorded observations regarding the weather； which formed the basis for some of the most epochal discoveries in meteorology； as we have seen。 But this was only a beginning。 The simple rain…gauge pointed the way to the most important generalization of the nineteenth century in a field of science with which； to the casual observer； it might seem to have no alliance whatever。 The wonderful theory of atoms； on which the whole gigantic structure of modern chemistry is founded； was the logical outgrowth； in the mind of John Dalton； of those early studies in meteorology。
The way it happened was this： From studying the rainfall； Dalton turned naturally to the complementary process of evaporation。 He was soon led to believe that vapor exists； in the atmosphere as an independent gas。 But since two bodies cannot occupy the same space at the same time； this implies that the various atmospheric gases are really composed of discrete particles。 These ultimate particles are so small that we cannot see themcannot； indeed； more than vaguely imagine themyet each particle of vapor； for example； is just as much a portion of water as if it were a drop out of the ocean； or； for that matter； the ocean itself。 But； again； water is a compound substance； for it may be separated； as Cavendish has shown； into the two elementary substances hydrogen and oxygen。 Hence the atom of water must be composed of two lesser atoms joined together。 Imagine an atom of hydrogen and one of oxygen。 Unite them； and we have an atom of water； sever them； and the water no longer exists； but whether united or separate the atoms of hydrogen and of oxygen remain hydrogen and oxygen and nothing else。 Differently mixed together or united； atoms produce different gross substances； but the elementary atoms never change their chemical naturetheir distinct personality。
It was about the year 1803 that Dalton first gained a full grasp of the conception of the chemical atom。 At once he saw that the hypothesis； if true； furnished a marvellous key to secrets of matter hitherto insolublequestions relating to the relative proportions of the atoms themselves。 It is known； for example； that a certain bulk of hydrogen gas unites with a certain bulk of oxygen gas to form water。 If it be true that this combination consists essentially of the union of atoms one with another （each single atom of hydrogen united to a single atom of oxygen）； then the relative weights of the original masses of hydrogen and of oxygen must be also the relative weights of each of their respective atoms。 If one pound of hydrogen unites with five and one…half pounds of oxygen （as； according to Dalton's experiments； it did）； then the weight of the oxygen atom must be five and one…half times that of the hydrogen atom。 Other compounds may plainly be tested in the same way。 Dalton made numerous tests before he published his theory。 He found that hydrogen enters into compounds in smaller proportions than any other element known to him； and so； for convenience； determined to take the weight of the hydrogen atom as unity。 The atomic weight of oxygen then becomes （as given in Dalton's first table of 1803） 5。5； that of water （hydrogen plus oxygen） being of course 6。5。 The atomic weights of about a score of substances are given in Dalton's first paper； which was read before the Literary and Philosophical Society of Manchester； October 21； 1803。 I wonder if Dalton himself； great and acu