106 
POPULAR SCIENCE REVIEW. 
passing round the magnet. Great heat is evolved when the concentrated 
focus of rays from a nearly hemispherical aluminium cup is deflected side- 
ways to the walls of the glass tube by a magnet. It may even be raised to 
the melting-point of platinum. 
Mr. Crookes speculates on the “ ultra-gaseous ” state of matter, in which, 
imder great rarefaction, the free path of a molecule is made so long that the 
hits in a given time may be disregarded in comparison to the misses, the pro- 
perties of “ gaseity ” are reduced to a minimum, and the matter exalted to a 
state in which very decided, but hitherto masked, properties come into play. 
This reveals a new world, wherein matter exists in a fourth state, where the 
corpuscular theory of light holds good, and where light does not always 
move in a straight line ; but which we must be content to observe and ex- 
periment on from the outside. 
Some Recent Acoustical Researches . — The harmonic overtones which ac- 
company a musical note are a well-known phenomenon, and their nature has 
been made pretty clear. There is another phenomenon presenting a certain 
analogy to this. It has lately been studied by a German physicist, Herr 
Auerbach, who applies to the notes generated the corresponding name of 
undertones. These undertones may be had by striking a tuning-fork vigor- 
ously, then placing its stem very lightly on a sounding-board. One hears 
the lower octave of the fundamental note of the tuning-fork. With, suitable 
materials, Herr Auerbach also obtains the lower fifth of the lower octave, 
and the lower fourth of this tone — that is, the double octave of the fork’s 
tone ; in fact, these resonance tones form a series of harmonic undertones. 
The phenomenon appears to depend essentially on the strength of the vibra- 
tions and the imperfect elasticity of the resonance-surface of the plate. Herr 
Auerbach has tried a variety of substances for undertones with tuning-forks. 
He finds that some, indeed most, substances give these tones ; that some give 
only a noise, as soon as the vibrations are moderately strong ; and some 
always give the tone of the fork, no matter how strongly this is sounded. 
Another German observer, Herr von Strouhal, has recently given some 
attention to a kind of tones not much studied hitherto — viz., those which 
arise when a rod is quickly swung through the air, or when currents of air 
impinge on stretched wires or sharp edges, &c. For pureness of tone, the 
swung rod must have all its parts moved with the same velocity, and it 
must be cylindrical. Ilerr Strouhal made an apparatus consisting of a ver- 
tical wooden column with two horizontal arms, between which the bodies to 
be forced through the air (mostly wires) were fixed, and he rotated the 
frame in its upright position at various speeds. Thus he got notes which 
rose in intensity and pitch with the speed. He found that the pitch of the 
“ friction tone ” (as he calls it) is independent of the tension of the wire, 
likewise of its length. But the length of the wire has a marked influence on 
the intensity of the tone. The longer the wire, the stronger (cceteris paribus) 
the tone. Further, the substance of the body is a matter of indifference ; 
but the height of the tone is directly proportional to the velocity of motion, 
and inversely so to the diameter of the wire. The author finds, moreover, 
that there is a way of making the “ friction tone ” produce the wire’s own 
tone — viz., when it is brought up to the same pitch with this (the wire 
being preferably thin and elastic), and similarly, by raising the pitch gradually 
