= Solution
Evaporation cools the remaining material when the escaping vapour takes <energy> from it faster than the surroundings supply <heat>. For an observable demonstration, compare two similar thermometer bulbs at the same initial <temperature>, one dry and the other wetted with a volatile <liquid>. Expose both to the same controlled airflow. The wetted bulb falls in <temperature>; the effect depends on vapour removal and environmental conditions. Another demonstration is continued heating during boiling at constant <pressure>: vapour is produced while the boiling <liquid>'s <temperature> remains nearly constant. The supplied <energy> has not vanished; it contributes to the <latent heat> and expansion associated with the <phase transition>.
One <calorimetry> method estimates the specific <latent heat of vaporization> by condensing a known mass $m_s$ of dry saturated steam at $T_s$ into a known mass $m_w$ of cooler <water>. Let $C_{\rm cal}$ be the vessel's <heat capacity>, and $T_i,T_f$ the initial and final water temperatures. Assuming complete condensation without escaped steam or thermal losses,
$$
m_s\{L_v+c_w(T_s-T_f)\}=(m_wc_w+C_{\rm cal})(T_f-T_i),
$$
so
$$
\boxed{L_v=\frac{(m_wc_w+C_{\rm cal})(T_f-T_i)}{m_s}-c_w(T_s-T_f).}
$$
The last term removes the sensible cooling of the condensed steam. Weighing the receiving vessel before and after gives $m_s$ only if splashing, evaporation and liquid carried in with the steam are controlled. A steam trap and an insulated transfer path address two distinct errors: droplets falsely counted as condensing steam, and cooling before the calorimeter.
The historical local application was <Richardson's ether spray>. A bellows atomizer dispersed <diethyl ether> into fine droplets, promoting rapid <vaporization> and withdrawal of <heat> from the surface; the cooling could produce local loss of sensation. <Benjamin Ward Richardson>'s 1866 account describes this evaporative method and its apparatus. Its historical physiological language is not adopted as a modern description of <nerve> signalling, and the account does not establish that the method is appropriate for every operation.<On a New and Ready Mode of Producing Local Anæsthesia by Benjamin Ward Richardson>[1866, description of the spray apparatus and cooling mechanism]{ref}
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