烧瓶、烧杯、锥形瓶等加热时需垫石棉网，目的是使受热容器受热均匀。为什么垫石棉网就能受热均匀？

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石棉的导热性相当好，相对于玻璃来说要好得多，你在加热的时候垫了石棉网后首先热量迅速在石棉网中传播，使真个石棉网的温度达到一致，这样他再传到玻璃器皿，玻璃器皿受热就是均匀的，如果直接加热那么玻璃器皿受热是从一个点受热再扩散的，受热当然不均了