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 proposed for the solar system. In one of his papers, Sir W. Herschel marshals the evidence which can be collected on this point. He arranges a selection from his observations on the nebulae in such a way as to give great plausibility to his view of the gradual transmutation of nebulae into stars. Herschel begins by showing us that there are regions in the heavens where a faint diffused nebulosity is all that can be detected by the telescope. There are other nebulae in which a nucleus can be just discerned, others again in which the nucleus is easily seen, and still others where the nucleus is a brilliant star-like point. The transition from an object of this kind to a nebulous star is very natural, while the nebulous stars pass into the ordinary stars by a few graduated stages. It is thus possible to exhibit a series of objects beginning at one end with the most diffused nebulosity and ending at the other with an ordinary fixed star or group of stars. Each object in the series differs but slightly from the object just before it and the object just after it. It seemed to Herschel that he was thus able to view the actual changes by which masses of phosphorescent or glowing vapour became actually condensed down into stars. The condensation of a nebula could be followed in the same manner as we can study the growth of the trees in the forest, by comparing the trees of various ages which the forest contains at the same time. In attempting to pronounce on the evidence with regard to Herschel’s theory, we must at once admit that the transmutation of a nebula into a star has never been seen. It is indeed very doubtful whether any changes of a nebula have ever been seen which are of the same character as the changes Herschel’s theory would require. It seems, however, most likely that the periods of time required for such changes are immense and that the changes accomplished in only a century or two are absolutely inappreciable.

The nebular theory is a noble speculation supported by plausible argument, and the verdict of science on the whole subject cannot be better expressed than in the words of S. Newcomb: “At the present time we can only say that the nebular hypothesis is indicated by the general tendencies of the laws of nature, that it has not been proved to be inconsistent with any fact, that it is almost a necessary consequence of the only theory by which we can account for the origin and conservation of the sun’s heat, but that it rests on the assumption that this conservation is to be explained by the laws of nature as we now see them in operation. Should any one be sceptical as to the sufficiency of these laws to account for the present state of things, science can furnish no evidence strong enough to overthrow his doubts until the sun shall be found growing smaller by actual measurement, or the nebulae be actually seen to condense into stars and systems.”

NECESSITAS (Gr.  ), in Orphic theology, the personification of absolute necessity. She appears as the mother of the Moerae (Fates), as the wife of Demiurgus (Fashioner of the World) and mother of Heimarmenē (Destiny). Her power is irresistible, even greater than that of the gods; to her was due the strife (battles with Titans, Giants) that raged amongst them of old, before the rule of love began; the world revolves round the spindle, which she holds in her lap. According to the Egyptian theory, she is one of the four deities present at the birth of every human being, her companions being the Daemon (guardian spirit), Tyche (Fortune) and Eros. On the citadel of Corinth there was a temple sacred to her and Bia (Violence), which none were permitted to enter. The Roman Necessitas is represented in the well-known ode of Horace (i. 35) as the forerunner and companion of Fortuna, holding in her brazen hand huge nails, a clamp and molten lead, symbolical of fixedness and tenacity.

 NECESSITY (Lat. necessitas), a term used technically in philosophy for the quality of inevitable happening; for example, hot air necessarily tends to rise. Thus it corresponds in the sphere of action to certainty in the sphere of knowledge. That the sun will rise to-morrow is a necessary event; and men anticipate the rising with certainty. In ordinary language the conception of necessity is rendered meaningless by being referred to the present or even to the past. A current definition of necessity is “the state which cannot be otherwise than it is.” Such a definition tells us nothing. How can any state be otherwise than it is? Necessity can have meaning only in reference to the future: it means absence of spontaneous power in that which acts necessarily. For the origin of the conception we must look to our inward personal experience of constraint. When we are acting under physical or mathematical or logical or moral necessity we are so far precluded from spontaneous action—in common phrase, we can do no otherwise—though the causes of constraint may be of very different kinds. In ethics the term necessitarianism is applied to that view of human action which regards all action as dictated by external causes (cf. ). The sense in which, if at all, the human mind can cognize necessity, i.e. causal connexion between events or states, has been the subject of vigorous discussion among philosophers. By sceptics and empiricists it is held that a law is merely a crystallized summary of observed phenomena. Thus J. S. Mill denies that a general proposition is more than an enumeration of particulars, and hence that syllogistic reasoning cannot amplify knowledge (see ). It is clear that the senses cannot apprehend causal connexion, and this impossibility gives rise to a prior conception according to which the conception of necessity is purely intellectual (see ).

 NECK (O. Eng. hnecca; the word appears in many Teutonic languages; cf. Dutch nek, Ger. Nacken; in O.&#x202f;E. the common word was heals; cf. Ger. Hals), that part of the body which connects the head with the trunk (see : Superficial and Artistic). The word is transferred to many objects resembling this part of the body in shape or function; it is thus applied to an isthmus, or to the narrowest portion of a promontory, to the narrow part of a musical stringed instrument connecting the head and body, as in the violin, or to a narrow pass between mountains, which in the Dutch form nek, appears in place-names in South Africa. In architecture, the “neck” is that part of the capital just above the “astragal,” and the term “necking” is applied to the annulet or round, or series of horizontal mouldings, which separates the capital of a column from the plain part or a shaft. In Romanesque work this is sometimes corded.

In Geology, the term “neck” is given to the denuded stump of an extinct volcano. Beneath every volcano there are passages of conduits up which the volcanic materials were forced, and after the mass has been levelled by denudation there is always a more or less circular pipe which marks the site of the crater. This pipe, which is filled with consolidated ashes or with crystalline lava, is the characteristic of a volcanic neck. Active volcanoes often stand on the sea-bottom and when the eruption comes to an end the volcano is slowly buried under layers of sediment. In tropical seas the coral animals cover over the submarine volcanoes which rise nearly to the surface and form great reefs of limestone around them. Should elevation take place after long ages the removal of the overlying strata will bring the volcanic mass to light, and in the normal course of things this will suffer denudation exactly like a recent volcano. Many instances of this are furnished by the geological history of the British Isles. In Carboniferous times, for example, before the Coal-measures were deposited, a shallow sea occupied the southern part of Scotland and the north of England. Volcanic activity broke out on the sea-bottom, and many volcanic cones, both small and large, were produced. These have long since been uplifted and the superjacent strata denuded away over a large part of the area which they occupied. In Derbyshire, Fife, the Lothians and the Glasgow district the remains of, Carboniferous volcanoes occur in every state of preservation. Some have the conical hills of lavas and ashes well preserved (e.g. Largo Law in Fifeshire); others retain only a small part of the original volcanic pile (e.g. Arthur’s Seat, Edinburgh; the Binn of Burntisland) and of the larger number nothing remains but the “neck” which shows where once the crater was situated.

In regions of former volcanic activity necks are the most persistent of all volcanic structures, because the active volcanic magma is located deep within the earth’s crust, and the pipe by which it rises to the surface is of great length and traverses a great thickness of