
_ granul 

_ Fepruary 10, 1923) ; 
t 
va 
NATURE 
203 

do not, as a rule, have the colloid coating. The 
_ plasticity of the clay depends upon the amount of 
Soha Agpieaa To separate the colloidal from the 
material, the clay is revolved at 40,000 
revolutions per minute in a separator. i 
Dr. Hubert Chatley, in a recent paper,’ has dis- 
cussed the properties of clay-mud, and states that 
it has three special features : ; 
(1) A granulated structure*of varying degrees of 
fineness. ‘ ; 
(2) A semi-permanent water content, which gives 
it peculiar mechanical properties. ; 
_ (3) A certain small reserve of chemical potential, 
which, under certain conditions, will cause 
it to change in various ways. 
He discusses the methods of observing the granular 
matter by means of the microscope, and states that 
_ the plasticity depends upon the size of the products 
® Society of Engineers, June 1922. 
and the proportion of colloids present. He divides 
the water content into three classes. 
Clay-mud containing 15 per cent. by weight of 
water has a tensile strength of*15 lb. per sq. inch, 
but doubling the water content reduces the tensile 
Strength to one-third of this amount. With 28 per 
cent. of water, its viscosity is about the same as a 
heavy grease, corresponding to a shear strain of 1 
radian per 100 seconds, under a shear stress of more 
than 100 grm. per sq. cm. It differs from heavy grease, 
however, in that water is extruded as the pressure is 
increased. It is not watertight, and dykes allow water 
to percolate very slowly, but if the surface of the 
dyke is dry, the surface tension may arrest the flow. 
The results of the data indicate agreement with 
common experience that the water content of clay 
is of great importance, and they also indicate that, 
as with all other materials, the working stresses 
should be within the “‘ elastic range.”’ 
Silvanus Thompson Memorial Lecture. 
At the request of the Finsbury Technical College 
Old Students’ Association, Sir Oliver Lodge 
gave the first of these lectures at the College on 
_ February 1, Sir Charles Parsons in the chair, to 
an audience numbering more than a thousand and 
including many eminent past students. After a 
reference to the splendid work of the College in the 
ast, and its hopes for the future, the lecturer recalled 
the brilliant succession of teachers—Ayrton, Perry, 
Meldola—colleagues of Thompson. Of the latter he 
said: ‘‘ The breadth of his outlook and width of his 
_ interests are almost proverbial ; his facility in foreign 
languages enabled him to hold his own in assemblies 
abroad, and he had a real artistic faculty. He had 
a love of discoveries in their nascent stages, and 
became a recognised historian of science. To a man 
of his.cosmopolitan feelings and pacific disposition, 
the war and its atrocities were a great distress ; 
grief and worry and overwork overtook him, and he 
succumbed on June 12, 1916—a victim of the war— 
. having been Anca am of Finsbury since 1885.” 
Proceeding to the subject of the lecture, a The 
Origins or Foundations of Wireless Communication,” 
and confining himself to matters prior to 1896, Sir 
Oliver recalled that the term “ inductance ’’ did not 
at first exist ; Lord Kelvin introduced it as a mathe- 
matical coefficient, Maxwell spoke of self-induction, 
and Heaviside originated the term now used. In the 
early work on the production and detection of electric 
waves in the ether, Kelvin, Maxwell, FitzGerald, and 
Hertz laid the foundations which made the present 
superstructure possible. Ney r 
n 1875 Edison observed the possibility of drawing 
sparks from insulated objects in the neighbourhood 
of an electrical discharge; already in 1842, Henry, 
in Washington, had surmised—through a similar 
observation—that there was some similarity between 
the etherial disturbance caused by the discharge of a 
conductor and the light emitted from an ordinary 
high-temperature source. Early in the ‘eighties 
David Hughes, working with the microphone and 
alvanometer, got something like a coherer, but was 
ie acnieed from pursuing the matter. In 1865 
Maxwell gave the theory of electric waves, before 
their generation or detection was understood ; he 
showed that they would travel with the velocity of 
light, that light was an electromagnetic phenomenon, 
that conductors of electricity must be opaque to 
light, and that the refractive index of’a substance 
_ was intimately related to its dielectric coefficient. 
NO. 2780, VOL. 111] 
This discovery aroused great enthusiasm, and one 
result was to influence the lecturer to devote his life 
to the study of electric waves; he discussed them 
with Fleming and FitzGerald, and spoke about them 
at the British Association in 1879 and later. In 1883 
FitzGerald proposed the generation of the waves by 
using the oscillatory discharge of a Leyden jar, and 
the lecturer, in 1887, produced and detected them. 
The waves were received on wires adjusted to the 
right length for resonance. The experiments of 
Hertz, who received the waves on a nearly closed 
ting’ of wire having a short spark gap, were reported 
by ‘FitzGerald at the British Association meeting of 
1888, and Sir Oliver calculated the horse-power of 
the oscillator—about 100, for a millionth of a second ‘ 
he exhibited many of the effects of the waves at the 
Royal Institution in 1889, but there was nothing 
akin to signalling; that was foreshadowed, in 1892, 
together with the possibility of tuning, by Sir William 
Crookes, who spoke of wave-lengths with which to 
signal to specific people, and alluded to Hughes's 
signals made from room to room without intervening 
wire. 
In 1890 Sir Oliver employed a form of coherer to 
complete a bell circuit, and in 1893 heard of Branly’s 
filings-coherer. In memory of Hertz, for whom Sir 
Oliver expressed the greatest admiration, both on 
account of his experimental skill and mathematical 
thoroughness, he gave a lecture at the Royal Institu- 
tion on the work of Hertz; at this lecture actual 
signalling with a coherer was demonstrated. This 
work led to the grant of Lodge’s patent in the United 
States, which was the fundamental patent of the 
American Marconi Company. The lecture stimu- 
lated Dr. Muirhead, Captain (now Admiral Sir Henry) 
Jackson, Admiral Popoff, Prof. Righi, and others to 
their experimental successes; in 1896 Mr. Marconi 
came to this country—and the rest is common 
knowledge. 
After the lecture the audience was entertained 
at a conversazione in the laboratories. A beautiful 
collection of Dr. Thompson’s paintings was on view, 
together with a number of his works, including a 
translation of Gilbert’s ““ De Magnete’”’ (1601) and 
a copy of the original. Coils constructed by Fara- 
day, the first Nicol prism, a coherer made by Sir 
Oliver Lodge, acoustical and optical models, and 
many personal relics were lent by the late Doctor’s 
family. Mr. W. M. Mordey, president of the Old 
Students’ Association of the College, gave a demon- 
