﻿Becker 
  — 
  Some 
  Queries 
  on 
  Mock 
  Differentiation. 
  29 
  

  

  will 
  occur 
  at 
  84 
  cm 
  from 
  the 
  original 
  contact. 
  After 
  10,000 
  

   years 
  the 
  distances 
  just 
  stated 
  will 
  each 
  be 
  multiplied 
  by 
  10. 
  

   At 
  the 
  expiration 
  of 
  a 
  million 
  years 
  the 
  water 
  would 
  be 
  just 
  

   sensibly 
  discolored 
  by 
  bluestone 
  at 
  350 
  m 
  and 
  semi-saturation 
  

   would 
  have 
  reached 
  to 
  a 
  distance 
  of 
  some 
  84 
  m 
  . 
  

  

  Viscosity 
  of 
  lavas. 
  — 
  Lavas 
  are 
  assuredly 
  far 
  less 
  diffusible 
  

   and 
  far 
  more 
  viscous 
  substances 
  than 
  sulphate 
  of 
  copper 
  solu- 
  

   tion.* 
  There 
  is 
  no 
  means 
  of 
  determining 
  with 
  any 
  approach 
  

   to 
  accuracy 
  what 
  the 
  diffusivity 
  of 
  lava 
  really 
  is, 
  but 
  there 
  is 
  

   some 
  reason 
  to 
  think 
  that 
  the 
  viscosity 
  of 
  even 
  the 
  most 
  fluid 
  

   lavas 
  is 
  more 
  than 
  50 
  times 
  as 
  great 
  as 
  that 
  of 
  water, 
  f 
  If 
  one 
  

  

  * 
  It 
  is 
  of 
  course 
  needless 
  to 
  call 
  attention 
  to 
  the 
  difference 
  between 
  fusibility 
  

   and 
  fluidity. 
  A 
  mass 
  may 
  be 
  easily 
  fusible 
  but 
  very 
  viscous 
  when 
  fused, 
  or 
  it 
  

   may 
  fuse 
  with 
  great 
  difficulty 
  and 
  when 
  fused 
  be 
  very 
  fluid 
  That 
  a 
  lava 
  is 
  very 
  

   fluid 
  does 
  not 
  indicate 
  that 
  it 
  is 
  considerably 
  superheated. 
  Water 
  at 
  0° 
  C. 
  is 
  

   sensibly 
  as 
  mobile 
  as 
  at 
  100°, 
  though 
  refined 
  experiments 
  reveal 
  a 
  difference. 
  

  

  \ 
  The 
  viscosity 
  of 
  lavas 
  is 
  evinced 
  by 
  the 
  slowness 
  with 
  which 
  lava 
  streams 
  

   advance. 
  Thus 
  in 
  the 
  Kilauea 
  eruption 
  of 
  184.0 
  the 
  lava 
  flowed 
  eleven 
  miles 
  

   down 
  a 
  declivity 
  of 
  1244 
  feet 
  in 
  two 
  days, 
  yet 
  according 
  to 
  Wilkes 
  and 
  Dana 
  

   (Characteristics 
  of 
  Volcanoes, 
  1890, 
  p. 
  63) 
  in 
  this 
  case 
  the 
  stream 
  was 
  fed 
  from 
  

   several 
  fissures 
  along 
  its 
  whole 
  course 
  iustead 
  of 
  being 
  an 
  overflow 
  from 
  a 
  single 
  

   opening. 
  The 
  heat 
  of 
  the 
  stream 
  must 
  have 
  been 
  pretty 
  well 
  maintained 
  by 
  such 
  

   accessions. 
  The 
  average 
  rate 
  of 
  flow 
  of 
  this 
  lava 
  down 
  a 
  2 
  per 
  cent 
  slope 
  was 
  

   about 
  £ 
  mile 
  per 
  hour 
  or 
  22 
  feet 
  per 
  mi 
  mite. 
  Now 
  water 
  in 
  a 
  stream 
  of 
  such 
  a 
  

   cross 
  section 
  on 
  such 
  a 
  grade 
  would 
  flow 
  at 
  about 
  6 
  miles 
  per 
  hour 
  or 
  about 
  24 
  

   times 
  as 
  fast. 
  Since 
  lava 
  is 
  about 
  2'5 
  times 
  as 
  dense 
  as 
  water, 
  these 
  data 
  roughly 
  

   indicate 
  for 
  the 
  kinetic 
  viscosity 
  25 
  x 
  24 
  or 
  60 
  times 
  the 
  viscosity 
  of 
  water. 
  

  

  In 
  the 
  case 
  of 
  gases 
  Maxwell 
  shows 
  that 
  the 
  diffus 
  vity 
  of 
  mass 
  is 
  1-5435 
  the 
  

   kinetic 
  viscosity 
  (Theory 
  of 
  Heat, 
  chapter 
  22) 
  and 
  that 
  the 
  ratio 
  of 
  diffusivity 
  to 
  

   viscosity 
  in 
  the 
  case 
  of 
  liquids 
  is 
  much 
  smaller 
  than 
  in 
  gases. 
  Hence 
  it 
  seems 
  

   safe 
  to 
  assume 
  the 
  diffusivity 
  of 
  lava 
  as 
  not 
  more 
  than 
  ^ 
  of 
  that 
  of 
  a 
  solution 
  

   like 
  that 
  of 
  bluestone. 
  

  

  In 
  the 
  more 
  recent 
  literature 
  I 
  have 
  not 
  met 
  with 
  investigations 
  which 
  throw 
  

   light 
  on 
  the 
  relations 
  between 
  diffusivity 
  and 
  viscusity. 
  The 
  resistance 
  which 
  

   molecules, 
  atoms 
  or 
  ions 
  meet 
  when 
  undergoing 
  diffusion 
  Ostwald 
  illustrates 
  by 
  

   the 
  slow 
  subsidence 
  of 
  pulverized 
  solids 
  in 
  air 
  (Lehrbuch 
  der 
  Allgem. 
  Chemie. 
  

   vol. 
  i, 
  1891, 
  p. 
  698). 
  This 
  slowness 
  is 
  due, 
  at 
  least 
  in 
  part, 
  to 
  the 
  viscosity 
  of 
  the 
  

   air. 
  and 
  Stokes 
  in 
  1851 
  showed 
  that 
  the 
  resistance 
  of 
  spherical 
  particles 
  is 
  pro- 
  

   portional 
  to 
  the 
  radius. 
  As 
  has 
  been 
  mentioned, 
  viscosity 
  is 
  a 
  resistance 
  

   due 
  to 
  the 
  diffusion 
  of 
  momentum. 
  That 
  viscosity 
  impedes 
  diffusion 
  of 
  

   matter 
  appears 
  evident, 
  for 
  example, 
  from 
  the 
  behavior 
  of 
  sealing 
  wax, 
  which 
  is 
  

   an 
  ultra-viscous 
  fluid. 
  Sticks 
  of 
  wax 
  of 
  different 
  colors 
  which 
  have 
  become 
  

   adherent 
  during 
  hot 
  weather 
  do 
  not 
  diffuse 
  into 
  one 
  another 
  sensibly 
  even 
  after 
  

   months 
  of 
  contact. 
  On 
  the 
  other 
  hand 
  diffusion 
  of 
  crystalloids 
  takes 
  place 
  in 
  

   quasi-solid 
  gelatine 
  jelly 
  at 
  the 
  same 
  rate 
  as 
  in 
  a 
  fluid 
  (Graham). 
  This, 
  however, 
  

   I 
  take 
  to 
  be 
  not 
  comparable 
  to 
  the 
  action 
  of 
  a 
  viscous 
  fluid, 
  "but 
  to 
  the 
  behavior 
  of 
  

   a 
  colloid 
  septum 
  such 
  as 
  bladder, 
  but 
  of 
  great 
  thickness. 
  The 
  jelly 
  seems 
  to 
  me 
  

   to 
  have 
  a 
  structure 
  similar 
  to 
  a 
  sponge 
  of 
  very 
  fine 
  grain 
  preventing 
  convection 
  

   but 
  not 
  the 
  diffusion 
  of 
  crystalloids. 
  Colloids 
  do 
  not 
  diffuse 
  in 
  such 
  a 
  jelly. 
  

  

  In 
  discussing 
  lavas 
  it 
  should 
  not 
  be 
  forgotten 
  that 
  high 
  temperature 
  accelerates 
  

   diffusion, 
  which 
  adds 
  to 
  the 
  difficulty 
  of 
  making 
  any 
  estimate 
  of 
  the 
  diffusivity 
  of 
  

   rock 
  magmas. 
  

  

  In 
  choosing 
  as 
  an 
  illustration 
  of 
  diffusion 
  an 
  hypothetical 
  magma 
  with 
  ^ 
  of 
  the 
  

   diffusivity 
  of 
  bluestone, 
  I 
  have 
  been 
  guided 
  in 
  part 
  by 
  observation 
  on 
  lavas. 
  

   Lavas 
  with 
  this 
  diffusivity 
  mingled 
  in 
  thin 
  layers, 
  like 
  banded 
  rhyo 
  lite, 
  would 
  dif- 
  

   fuse 
  into 
  approximate 
  uniformity 
  in 
  a 
  few 
  hours. 
  No 
  one 
  can 
  doubt 
  that 
  the 
  

   rhyolitic 
  bands 
  have 
  been 
  in 
  contact 
  for 
  at 
  least 
  a 
  few 
  hours 
  in 
  the 
  fluid 
  state 
  and 
  

   that 
  they 
  must, 
  therefore, 
  be 
  less 
  diffusible 
  than 
  my 
  hypothetical 
  lava. 
  Similar 
  

   banding 
  is 
  not 
  infrequent 
  in 
  andesites 
  though 
  it 
  is 
  less 
  common 
  than 
  in 
  rhyolite. 
  

  

  