MOLECULE 



MOLESCHOTT 



255 



to have a sheet of matter thinner than from 

 Jmnrj cm. to nroVinni s cm- thick; and that a 

 figure of this order represents the diameter of the 

 space occupied by one molecule. From tlie chemical 

 point of view, the lawH of definite proportion and of 

 multiple proportion find their best explanation in 

 the atomic theory ; but the chemical atom, which 

 is active, is not the same thing as the physical 

 molecule, which is chemically somewhat inert ; as 

 we see, for example, in the contrast afforded by 

 the mutual combination of oxygen and hydrogen 

 when in the nascent state, or when exposed to 

 high temperature, and their want of combination 

 when simply mixed as free gases at ordinary tem- 

 peratures. 



The kinetic theory of gases that gases consist 

 of molecules in a continual condition of mutual 

 collision and rebound leads to the theorem ( Avoga- 

 dro's Law) that all gases contain within the same 

 volume, under equal conditions of temperature and 

 pressure, the same number of molecules ; and if we 

 assume each molecule to lie made up of the masses 

 of a group of chemical .atoms sufficient to make up 

 the smallest possible free mass of the chemical sub- 

 stance, then this law is in perfect accord with the 

 observations of Dulong and Petit on the relative 

 densities of gases, which are proportional to the 

 weights of the molecules, as derived from the known 

 atomic weight* or combining masses of the elements 

 (see CHEMISTRY). 



For this reason it has lieen usual to ascertain the 

 molecular weight of substances by observation of 

 their gas or vapour density, the molecular weight 

 of hydrogen l>eing reckoned as 2, or that of oxygen 

 as 32. Thus, hydrochloric acid gas, which is 18"2o 

 times as heavy a.s hydrogen gas, has a molecular 

 weight 36-5, which is equal to the sum of the 

 atomic weights of chlorine ( = 3o'5 ) and of hydrogen 

 (=1). Hydrogen itself has a molecular weight 

 - 2, not = 1, liecause its molecule is held to lie 

 made up of two atoms, each of which has an 

 atomic weight equal to unity ; and so with other 

 elementary gaseous or volatile snlwtances. There 

 are abnormalities, however ; sulphur vapour has so 

 great a density as to show its molecule to consist, 

 below a red heat, of six atoms, while at higher 

 temperatures it consists of two ; iodine vapour at 

 lower temperatures has two, at higher one atom 

 in the molecule; mercury, cadmium, zinc, potas- 

 sium, and sodium molecules contain one atom 

 each ; chlorine and bromine molecules partially 

 break up into single atoms when heated. The 

 molecules of liquids are probably compound ; those 

 of sol ills are almost certainly so, as allotropic 

 modifications and variations in crystalline form 

 show, though the vapours, produced from these 

 various forms, if the solids be volatile, are identical. 

 Other and more convenient means of measuring 

 molecular weight* have recently become known. 

 Thr-c are (1) a solid dissolved in a liquid lowers 

 the ya|Miur pressure (or 'vapour tension') of that 

 liquid by an amount which liears to the original 

 vapour pressure at the particular temperature of 

 cxiicriiiieut the same ratio as the number of mole- 

 cules of the dissolved substance does to the total 

 nmnlier of molecules in the solution (Kaoult); 

 (2) the freezing-point of a solution is lowered in a 

 similar ratio (Kaoult); (3) osmotic pressure (see 

 OS.MOSK) is the same as that which would be exer- 

 ri-i'd by the substance dissolved if it were trans- 

 formed into a 'gas' and made to occupy alone the 

 space occupied by the solution (Van t'Hotf); whence 

 the specific gravity of the ' gns ' can lie ascertained 

 and the molecular weight of the dissolved sub- 

 MUM computed. In all these canes there are 

 abnormalities observed, which are due to dissocia- 

 tion or breaking up of the molecules; these are 

 specially observed m the case of salts and other 



electrolytic substances. A solution of common 

 salt, NaCl, for example, contains (Arrhenius) very 

 little combined chloride of sodium, the particles of 

 which have split up into monatomic sub-molecules 

 of sodium and chlorine, heavily charged with oppo- 

 site electricities ; and the electrolytic conductivities 

 afford additional means of measuring the proportion 

 of the salt which has thus dissociated. Those acids 

 which thus dissociate most into active sub-mole- 

 cules are the most active. Heat has also a power- 

 ful action in breaking up molecules, and causing 

 either dissociation, as in the case of acetic acid, 

 whose molecules are complex until its vapour is 

 strongly heated ; or chemical decomposition, as in 

 many well known reactions. Those compounds 

 whose formation is attended with the evolution of 

 heat have molecules which are generally most 

 stable at low temperatures; those which' absorb 

 heat dining their formation are, as a rule, most 

 stable at high temperatures. When a substance 

 is heated, the energy imparted to the molecules is 

 not only s)>ent in giving them motion relatively to 

 one another, but also, and in many instances to a 

 still greater extent, in giving the molecules them- 

 selves movements of elastic vibration and of rota- 

 tion ; and this distnrlied condition affects the 

 stability of the molecule. How the molecules are 

 built uii from their constituent molecules in each 

 particular snlistance and its isomers is a matter 

 now being diligently investigated ; the graphic 

 formnlie to be found in all the text-books of 

 chemistry, and which have been of enormous 

 utility iu the systematisation and discovery of 

 organic compounds, are lieginning to give place to 

 ' slereochemical ' formula-, in which the attempt is 

 made to represent the tridimensional relations of 

 the atoms within the molecule. The forces be- 

 tween molecules vary with their mutual proximity 

 (see MATTER) ; and many of the observed anoma- 

 lies in the obedience of Gases (q.v.) to Boyle's 

 Law under varying pressures and temperatures 

 can be reduced to order by taking instead of v, the 

 whole volume occupied by the gas, a term v - *, 

 the free space within that volume unoccupied liy 

 the molecules of the gas ; and instead of p, the 



external pressure, a term p + where ^ repre- 

 sents a force of mutual attraction of the molecules. 

 See Ostwald's General Chemistry (1890). 



Mole-rat (.V/z), a genus of rodent quad- 

 rupeds of the family Mnridic, having teeth almost 

 like those of rats, but in many respects resembling 

 moles, as in general form, shortness of limbs, con- 

 cealment of ears, snmllness or even rudimentary 

 condition of eyes, and burrowing habits although 

 their food is altogether different, consisting wholly 

 of vegetable sulistances, and chiefly of roots. The 

 mole-rats are almost confined to the African con- 

 tinent and to the oriental region ; a few species 

 occur in south east Russia, Greece, and Hungary. 

 Bathytrgvi maritimns of the Cape is a large species 

 which inhabits the sand-dunes of the coast. 



Molrsrliot t. JAKOB, physiologist, born at Bois- 

 le-Diic in Holland, 9th August 1822, studied medi- 

 cine at Heidelberg, and taught there physiology, 

 anatomy, and anthropology from 1847 until 1854, 

 when he resigned his chair, the senate of the univer- 

 sity having ' warned ' him on account of the strong 

 materialistic tendency of his writings. In 1853 he 

 established a private laboratory and worked in it 

 until 185<i, when he was nominated professor of 

 Physiology at Zurich ; in 1861 he moved to the 

 university of Turin, and in 1878 to that of Rome, 

 where he died 20th May 1893. He wrote some 

 twenty works, in German and Italian, on various 

 branches of physiological research, including one on 

 the Natural 11 istury of Man and Animals (1855). 



