This is of special importance in connection with phenomena we have presently to consider.  It means that, knowing the chemical composition and density of any medium whatsoever, solid, liquid or gaseous, we can calculate accurately the distance to which any particular alpha ray will penetrate.  Nor have the temperature and pressure to which the medium is subjected any influence save in so far as they may affect the proximity of one atom to another.  The retardation of the alpha ray in the atom is not affected.

This valuable additive law, however, cannot be applied in strictness to the amount of ionisation attending the ray.  The form of the molecule, or more generally its volume, may have an influence upon this.  Bragg draws the conclusion, from this fact as well as from the notable increase of ionisation with loss of speed, that the ionisation is dependent upon the time the ray spends in the molecule.  The energy of the ray is, indeed, found to be less efficient in producing ionisation in the smaller atomm.

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Before leaving our review of the general laws governing the passage of alpha rays through matter, another point of interest must be referred to.  We have hitherto spoken in general terms of the fact that ionisation attends the passage of the ray.  We have said nothing as to the nature of the ionisation so produced.  But in point of fact the ionisation due to an alpha ray is sui generis.  A glance at one of Wilson’s photographs (Fig. 14.) illustrates this.  The white streak of water particles marks the path of the ray.  The ions produced are evidently closely crowded along the track of the ray.  They have been called into existence in a very minute instant of time.  Now we know that ions of opposite sign if left to themselves recombine.  The rate of recombination depends upon the product of the number of each sign present in unit volume.  Here the numbers are very great and the volume very small.  The ionic density is therefore high, and recombination very rapidly removes the ions after they are formed.  We see here a peculiarity of the ionisation effected by alpha rays.  It is linear in distribution and very local.  Much of the ionisation in gases is again undone by recombination before diffusion leads to the separation of the ions.  This “initial recombination” is greatest towards the end of the path of the ray where the ionisation is a maximum.  Here it may be so effective that the form of the curve is completely lost unless a very large electromotive force is used to separate the ions when the ionisation is being investigated.

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