The four kinds of atoms are earth, water, fire, and air atoms. These have mass, number, weight, fluidity (or hardness), viscosity (or its opposite), velocity, characteristic potential colour, taste, smell, or touch, not produced by the chemical operation of heat. Akas'a (space) is absolutely inert and structure-less being only as the substratum of sound, which is supposed to travel wave-like in the manifesting medium of air. Atomic combination is only possible with the four elements. Atoms cannot exist in an uncombined condition in the creation stage; atmospheric air however consists of atoms in an uncombined state.
Two atoms combine to form a binary molecule (_dvya@nuka_). Two, three, four, or five dvya@nukas form themselves into grosser molecules of trya@nuka, catura@nuka, etc. [Footnote ref 483]. Though this was the generally current view, there was also another view as has been pointed out by Dr B.N. Seal in his _Positive Sciences of the Ancient Hindus_, that the "atoms have also an inherent tendency to unite," and thatthey do so in twos, threes, or fours, "either by the atoms falling into groups of threes, fours, etc., directly, or by the successive addition of one atom to each preceding aggregate [Footnote ref 484]." Of course the atoms are regarded as possessed of an incessant vibratory motion. It must however be noted in this connection that behind this physical explanation of the union of atoms there is the ad@r@s@ta, the will of Is'vara, which gives the direction of all such unions in harmony with the principle of a "moral government of the universe," so that only such things are produced as can be arranged for the due disposal of the effects of karma. "An elementary substance thus produced by primary atomic combination may however suffer qualitative changes under the influence of heat (_pakajotpatti_)" The impact of heat corpuscles decomposes a dvya@nuka into the atoms and transforms the characters of the atoms determining them all in the same way. The heat particles continuing to impinge reunite the atoms so transformed to form binary or other molecules in different orders or arrangements, which account for the specific characters or qualities finally produced. The Vais'e@sika holds that there is first a disintegration into simple atoms, then change of atomic qualities, and then the final re-combination, under the influence of heat. This doctrine is called the doctrine of _pilupaka_ (heating of atoms). Nyaya on the other hand thinks that no disintegration into atoms is necessary for change of qualities, but it is the molecules which assume new characters under the influence of heat. Heat thus according to Nyaya directly affects the characters of the molecules and changes their qualities without effecting a change in the atoms. Nyaya holds that the heat-corpuscles penetrate into the porous body of the object and thereby produce the change of colour. The object as a whole is not disintegrated into atoms and then reconstituted again, for such a procedure is never experienced by observation. This is called the doctrine of _pi@tharapaka_ (heating of molecules). This is one of the few points of difference between the later Nyaya and Vais'e@sika systems [Footnote ref 485].
Chemical compounds of atoms may take place between theatoms of the same bhuta or of many bhutas. According to the Nyaya view there are no differences in the atoms of the same bhuta, and all differences of quality and characteristics of the compound of the same bhuta are due only to diverse collocations of those atoms. Thus Udyotakara says (III. i. 4) that there is no difference between the atom of a barley seed and paddy seed, since these are all but atoms of earth. Under the continued impact of heat particles the atoms take new characters. It is heat and heat alone that can cause the transformations of colours, tastes etc. in the original bhuta atoms. The change of these physical characters depends on the colours etc. of the constituent substances in contact, on the intensity or degree of heat and also on the species of tejas corpuscles that impinge on the atoms. Heat breaks bodies in contact into atoms, transforms their qualities, and forms separate bodies with them.
Pras'astapada (the commentator of Vais'e@sika) holds that in the higher compounds of the same bhuta the transformation takes place (under internal heat) in the constituent atoms of the compound molecules, atoms specially determined as the compound and not in the original atoms of the bhuta entering into the composition of the compound. Thus when milk is turned into curd, the transformation as curd takes place in the atoms determined as milk in the milk molecule, and it is not necessary that the milk molecule should be disintegrated into the atoms of the original bhuta of which the milk is a modification. The change as curd thus takes place in the milk atom, and the milk molecule has not to be disintegrated into k@siti or ap atoms. So again in the fertilized ovum, the germ and the ovum substances, which in the Vais'e@sika view are both isomeric modes of earth (with accompaniments of other bhutas) are broken up into homogeneous earth atoms, and it is these that chemically combine under the animal heat and biomotor force vayu to form the germ (_kalala_). But when the germ plasm develops, deriving its nutrition from the blood of the mother, the animal heat breaks up the molecules of the germ plasm into its constituent atoms, i.e. atoms specifically determined which by their grouping formed the germ plasm. These germ-plasm atoms chemically combine with the atoms of the food constituents and thus produce cells and tissues [Footnote ref 486]. This atomic contact is called _arambhaka-sa@myoga_.
In the case of poly-bhautik or bi-bhautik compounds there is another kind of contact called _upa@s@tambha_. Thus in the case of such compounds as oils, fats, and fruit juices, the earth atoms cannot combine with one another unless they are surrounded by the water atoms which congregate round the former, and by the infra-atomic forces thus set up the earth atoms take peculiar qualities under the impact of heat corpuscles. Other compounds are also possible where the ap, tejas, or the vayu atoms form the inner radicle and earth atoms dynamically surround them (e.g. gold, which is the tejas atom with the earth atoms as the surrounding upa@s@tambhaka). Solutions (of earth substances in ap) are regarded as physical mixtures.
Udayana points out that the solar heat is the source of all the stores of heat required for chemical change. But there are differences in the modes of the action of heat; and the kind of contact with heat-corpuscles, or the kind of heat with chemical action which transforms colours, is supposed to differ from what transforms flavour or taste.
Heat and light rays are supposed to consist of indefinitely small particles which dart forth or radiate in all directions rectilineally with inconceivable velocity. Heat may penetrate through the interatomic space as in the case of the conduction of heat, as when water boils in a pot put on the fire; in cases of transparency light rays penetrate through the inter-atomic spaces with _parispanda_ of the nature of deflection or refraction (_tiryag-gamana_). In other cases heat rays may impinge on the atoms and rebound back--which explains reflection. Lastly heat may strike the atoms in a peculiar way, so as to break up their grouping, transform the physico-chemical characters of the atoms, and again recombine them, all by means of continual impact with inconceivable velocity, an operation which explains all cases of chemical combination [Footnote ref 487]. Govardhana a later Nyaya writer says that paka means the combination of different kinds of heat. The heat thatchanges the colour of a fruit is different from that which generates or changes the taste. Even when the colour and taste remain the same a particular kind of heat may change the smell. When grass eaten by cows is broken up into atoms special kinds of heat-light rays change its old taste, colour, touch and smell into such forms as those that belong to milk [Footnote ref 488].
In the Nyaya-Vais`e@sika system all action of matter on matter is thus resolved into motion. Conscious activity (_prayatna_) is distinguished from all forms of motion as against the Sa@mkhya doctrine which considered everything other than puru@sa (intelligence) to arise in the course of cosmic evolution and therefore to be subject to vibratory motion.