8 NATURE 
value for the charge, and normal values for absorption, 
etc., while A/2 does not. 
And (iii) on P, or some power of it depend (see table) 
(1) the percentage of the incident 8 radiation reflected 
for UrX, RaE, Act D); (2) the absorption of kathode, 
or B rays (see Lenard, Ann. d. Phys., vol. lvi., p. 275, 
1895, and Crowther, Phil Mag., vol. xii., p. 379, 1906); 
(3) the number of nuclear electrons and f particles 
ejected (see Soddy, Jahrb. Radioakt. und Elektronik, 
1913, vol. x., p. 193); (4) quanta of energy the 6 par- 
ticle looses on Rutherford’s theory (Phil Mag., vol. 
XXVi., p. 717, 1913) in traversing the non-nuclear elec- 
trons of the atom it came from; (5) probably the 
decrease of velocity of a particles traversing matter 
(Bohe! Pik Mag, vol. (xxv., p. 27, 1913). ~ The table 
gives the number of electrons, causing this decrease, 
in approximate values. 
= | pb TOM |Perc.Refl. Brad., Perc. Refl. @rad./P* N, 
bD NP2 |UrX RaE AcD|} UrX RaK AcD | Bohr 
| 
Al | rx 27° *30, 38482 «9:0 4(1175) 104 
Me |-24 | 6'4- 6°2 7|-41 141. 47+) 8:4. 84° -9°6 
Ni | 24 43) 143,48 819, 8°90, 19°38 
Gun 25) 0 SanOO. Nas iA Se aes 19:0 )) TOA 
Pe 20 N n69550°7 al AS) A5 5 Balh ob) 8.9 ~ 1078 
Agaleqtal)  Si28 503 val 655057 Cone’ 90 99 
Sn 4405 104 1157.02: mior| 37 9'4, “1075: 7-38 
Pi 42500 1 O84h 1559-4) 00), 0879/88" OX 104 
IA Gop NP Geigy oss |) WS CS 7S) ORO eG) aio) 61 
Pb |,60)| 10:8 6°2."| 68" Fo 0 | 8°38 gl 10°3 65 
Bi | 61 OMe OLE OLOmer Onlan elOr 
| 6°2 8°67 8:98 10°19 
From (ii, 2) n=c/A=2-465.10'°(M—1)?; from (ii, 4) 
Vinin=2'24-10°(M—1). Now Vinin>v of the electron 
giving K-radiation, and if this =1-93.10°(M—1), then 
mv? /2n =0-88. 1-93.710— *7/\2.2-465.10 =6-62.10—77 = 
Planck’s h. 
A. VAN DEN BROEK. 
Gorsel, Holland, February 9. 
An Early Slide Rule. 
Dre Morean, in article ‘‘Slide Rule”? in the Penny 
Cyclopaedia, points out that though Gunter first used 
a logarithmic scale, the real inventor of the logarithmic 
slide was Oughtred. ‘‘In the year 1630 he showed 
it to his pupil, William Forster, who obtained his con- 
sent to translate and publish his own description of 
the instrument, and rules for using it. This was done 
under the following title, ‘The Circles of Proportion 
and the Horizontal Instrument,’ London, 1632; fol- 
lowed in 1633, by an ‘ Addition, etc.,’ with an appendix 
having title, ‘ The Declaration of the two Rulers for 
Calculation.’”’ After referring to a republication of 
this work in 1660, he goes on :—‘‘ The next writer 
whom we can find is Seth Partridge, in a ‘ Description, 
etc., of the Double Scale of Proportion,’ London, 1685. 
He studiously conceals Oughtred’s name; the rulers 
of the latter were separate, and made to keep together 
in sliding by the hand; perhaps Partridge considered 
the invention his own, in right of one ruler sliding 
between two others kept together by bits of brass.” 
Prot.) Cajori, in his ibook, “SA (History of the 
Logarithmic Slide Rule,” 1909, the result of an ex- 
haustive inquiry into the literature of the subject, 
quotes De Morgan, and continues (p. 17), ‘‘To 
Partridge we owe, then, the invention of the slide.” 
In an addendum (p. vi.), and in Nature, February 24, 
NOwegId, VOL. 93] 
[MarcH 5, 1914 
1910, p. 489, he refers to a copy of Partridge’s book 
in his own possession, published in 1662, in which it 
is stated that the book was written in 1657. 
Dr. Alexander Russell, in NaTurE, January 30, 1910, 
p. 307 states:—‘‘A few years before 1671, Seth 
Partridge rediscovered the sliding principle, perfected 
it, and gave an almost complete specification for the 
slide rule which is used to-day by engineers. . . . Per- 
sonally, I consider that Seth Partridge is the real 
inventor of the modern ro-in. slide rule.” 
My object in writing is to direct attention to the 
fact that there is in the Science Museum at South 
Kensington a slide rule which is inscribed, ‘t‘ Made 
by Robert Bissaker for T. W., 1654.’’. This proves 
that the slide was invented and in use three years 
before Partridge wrote his pamphlet, and eight years 
before the earliest known date of its publication. 
This very early example of the instrument is of 
boxwood, well made, and bound together with brass 
at the two ends. It is of the square type, a little 
more than 2 ft. in length, and bears the logarithmic 
lines first described by Edmund Gunter. Of these, the 
num., sin., and tan lines are arranged in pairs, 
identical and contiguous, one line in each pair being 
on the fixed part, and the other on the slide. As Seth 
Partridge describes no feature which is not embodied 
in this example of the instrument, it would appear 
that less credit is due to him for invention in connec- 
tion with the slide rule than has hitherto been given. 
In this year of the Napier tercentenary celebration 
it is interesting to know that a slide rule is still in 
existence which was made only forty years after the 
invention of logarithms. Davip BAXANDALL. 
The Science Museum, South Kensington, S.W. 
The Permeability of Echinoderm Eggs to Electrolytes. 
In 1910 J. F. McClendon showed that the electrical 
conductivity of echinoderm eggs is considerably in- 
creased after fertilisation, and inferred from this fact 
that the act of fertilisation causes an increase in the 
permeability of the egg-surface to electrolytes. In his 
recent book (‘‘ Artificial Parthenogenesis and Fertilisa- 
tion’’) Prof. Loeb suggests that the increase in con- 
ductivity is not due to an increase in permeability, 
but would be produced “if in consequence of mem- 
brane-formation the degree of electrolytic dissociation 
of the surface film of the egg should be increased ”’ 
(p. 122). 
I have recently found that the electrical conductivity 
of unfertilised and fertilised eggs is very greatly 
affected by the presence of very low concentrations of 
simple trivalent positive ions; a concentration of 
0-0002M Ce: decreases the conductivity of the unfer- 
tilised eggs of Sphaerechinus granularis by as much 
as 40 per cent. Such solutions likewise affect the 
conductivity of the fertilised eggs, but to a less degree. 
Whereas it is almost inconceivable that these pheno- 
mena are due to a decrease in the electrolytic con- 
tents of the surface-film of the egg, I have found 
considerable evidence in support of the suggestion 
that the electrical conductivity of these eggs is deter- 
mined, at least partially, by the charge on the egg- 
surface. As Perrin, Girard, and Mines have shown, 
this factor also determines the degree of permeability 
of membranes to electrolytes. In short, McClendon’s 
original contention, that the increase in electrical con- 
ductivity of eggs after fertilisation is due to an in- 
creased permeability of the egg-surface, is very much 
more satisfactory than Prof. Loeb’s suggestion. 
J. Gray. 
Stazione Zoologica, Napoli, Italy. 
February 5. 
