error C2664: “find_char”: 不能将参数 1 从“const char [14]”转换为“std::string &

#include <iostream>
#include <string>
using std::cout;
using std::endl;
using std::string;
//const引用形参举例
//非const引用形参只能与完全同类型的非const对象关联
std::size_t find_char(string &s,char c)
{
   string::size_type i = 0;
   while(i != s.size() && s[i] != c)
       ++i;
   if(i == s.size())
       return string::npos;
   else
       return i;
}
int main(int argc, char *argv[])
{
    //字面值常量为const对象，调用出错
    if(find_char("Hello, world.",'.') != string::npos)
    {
        cout<<"a sentence."<<endl;
    }
    return 0;
}