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POPULAR SCIENCE REVIEW. 
cylinder containing the ends of two pistons, which act by leverage upon the- 
brakes, the admission of the air being effected by means of a reduction of 
pressure, which opens a set of valves communicating with the cylinder. On 
its entrance it pushes the pistons out, and these then act upon the brakes. 
A further advantage is said to be that in case of the train breaking into two 
or more parts, the brakes will immediately act automatically and bring the 
carriages to a standstill. This brake is already in use on the Midland Rail- 
way. 
The experiments above alluded to, with a hollow elastic ball, were made by 
means of a nozzle attached to the apparatus, through which the compressed 
air could be allowed to escape. When the ball is placed within the current 
of air, with the tap upright, it is sustained as if by a jet of water, but shows 
little or no revolving motion. When the tap is inclined at an angle of 30° 
or 40°, the ball is still supported by the current, but it recedes and advances 
in the line of the tap and revolves so rapidly as to cause a distinct flattening 
of its poles, and this goes on until the ball becomes almost a disc. The 
axis of rotation then gradually changes until it is at right angles to its original 
position, when the speed of rotation diminishes, and the ball gradually 
ceases to revolve; then it begins rotating upon its new axis, and passes, 
through the same series of changes repeatedly so long as it is exposed to the 
action of the jet. 
MINERALOGY. 
The Accidental Coloration of the Zeolites . — The coloured varieties of these 
minerals may be divided into two groups : those owing their colour to 
the presence of inorganic material like the red zeolites of Dumbarton and 
the Fassathal, which enclose hydrated oxide “of iron, and those which 
contain organic compounds. Webskv (“ Jahrbuch fur Mineralogie,” 1878, 
75) has published a memoir on the mineral species which are included in the 
second category. The best known minerals of this class are the brown desmin 
and heulandite occurring in the deposits of magnetite at Arendal ; here the 
colour is unquestionably due to the presence of an asphalt-like substance. 
Similar crystals have recently been met with in druses in the granite of 
Graben. The desmin of Striegau exhibits pale leather-brown and wood- 
brown hues. The rare crystals of chabasite have an orange red approaching 
a chestnut-brown colour, very intense on their surface, lighter but still 
relatively intense in the interior. Chabasite appears to take up the largest 
amount of the colouring substance. A fragment of this mineral when 
heated in a closed tube turns black, a small amount of a tar-like substance 
distilling off. The mineral is afterwards found to have a pale grey colour ; 
if heated for a longer time in an open tube the colour disappears, at least 
from the surface. Two specimens of heulandite from the granite of Striegau,. 
now in the Berlin collection, exhibited two years ago a fine citron yellow 
colour, now they are much paler ; others are referred to which were covered 
with perfectly colourless crystals of laumonite. It is curious to find that 
minerals, like chabasite, desmin, heulandite, and laumonite, which agree so 
closely as regards their elementary constituents, and differ only in the pro- 
portion of these ingredients, should exhibit a very great disparity in their 
