Every one who has made friction experiments knows how unsatisfactory and inconsistent they often are.  We can only discuss notable quantities and broad results, unless the most conscientious care be taken to eliminate errors.  The net result here is that ice at about -10 deg.  C. when pressed on by a very light weight possesses a

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coefficient of friction comparable with the usual coefficients of solids on solids, but when the pressure is increased, the coefficient falls to about half this value.

The following table embodies some results obtained on the friction of ice and glass, using the methods I have shown you.  I add some of the more carefully determined coefficients of other observers.

Wt. in      On Plate         On Ice         On Ice
Grams.       Glass.           at 0 deg.  C.       at 10 deg.  C.

Angle.  Coeff.     Angle.  Coeff.   Angle.  Coeff
Aluminium   2.55    121/2 deg.  0.22        12 deg.   0.21     131/2 deg.   0.24
Same      155       121/2 deg.   0.22        6 deg.   0.11       7 deg.   0.12
Brass       6.5     121/2 deg.  0.22        10 deg.   0.17     101/2 deg.   0.18
Same      107       121/2 deg.   0.22        5 deg.   0.09       6 deg.   0.10

Steel on steel (Morin) — — — — 0.14
Brass on cast iron (Morin) — — 0.19
Steel on cast iron (Morin) — — 0.20
Skate on ice (J.  Mueller) — — — 0.016—­0.032
Best-greased surfaces (Perry) — 0.03—­0.036

You perceive from the table that while the friction of brass or aluminium on glass is quite independent of the weight used, that of brass or aluminium on ice depends in some way upon the weight, and falls in a very marked degree when the weight is heavy.  Now, I think that if we had been on the look out for any abnormality in the friction of hard substances on ice, we would have rather anticipated a variation in the

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other direction.  We would have, perhaps, expected that a heavy weight would have given rise to the greater friction.  I now turn to the explanation of this extraordinary result.

You are aware that it requires an expenditure of heat merely to convert ice to water, the water produced being at the temperature of the ice, i.e. at 0 deg.  C., from which it is derived.  The heat required to change the ice from the solid to the liquid state is the latent heat of water.  We take the unit quantity of heat to be that which is required to heat 1 kilogram of water 1 deg.  C. Then if we melt 1 kilogram of ice, we must supply it with 80 such units of heat.  While melting is going on, there is no change of temperature if the experiment is carefully conducted.  The melting ice and the water coming from it remain at 0 deg.  C. throughout the operation, and neither the thermometer nor your own sensations would tell you of the amount of heat which was flowing in.  The heat is latent or hidden in the liquid produced, and has gone to do molecular work in the substance.  Observe that if we supply only 40 thermal units, we get only one-half the ice melted.  If only 10 units are supplied, then we get only one eighth of a kilogram of water, and no more nor less.